diff --git a/scripts/add-ons/README b/scripts/add-ons/README
new file mode 100644
index 000000000..aa308364f
--- /dev/null
+++ b/scripts/add-ons/README
@@ -0,0 +1,16 @@
+External Scripts
+----------------
+
+Add-on scripts are executed vi an alias mechanism in taskwarrior.  This has the
+effect of this command:
+
+    task <script> <arguments>
+
+resulting in the ultimate execution of:
+
+    <script> <arguments>
+
+All external scripts are treated in the same manner, and have no access to the
+taskwarrior internals.
+
+---
diff --git a/scripts/extensions/README b/scripts/extensions/README
index 8f4bcac02..1b7e24e74 100644
--- a/scripts/extensions/README
+++ b/scripts/extensions/README
@@ -1,8 +1,100 @@
-There are several types of extension:
+Extensions
+----------
 
-- Hook-based
-- User Defined Attribute
-- Command
-- Format
-- DOM
+Extensions are Lua scripts that require installation and configuration, and when
+invoked have access to taskwarrior internals through a Lua API.
+
+There are several types of extension.  Each type has different requirements, and
+is called in different ways.
+
+All extensions must be installed, which means they must implement the 'install'
+function, which provides:
+
+  - Type              One of: program, task, uda, command, format, dom
+  - Name              Single word name
+  - Version           Version string
+  - Description       One to two line description
+  - Author            Author's name
+  - Contact           Author's contact
+  - License           Distribution License
+  - Copyright         Copyright notice
+
+
+Program Hooks
+-------------
+
+These are scripts that are triggered by program-level events.  The supported
+program hooks are:
+
+    on-launch         As soon as is convenient after program launch
+    on-exit           As late as possible while still providing API
+    on-file-read      Immediately before reading a data file
+    on-file-write     Immediately after writing a data file
+    on-synch          Immediately before a synch operation
+    on-merge          Immediately before a merge operation
+    on-gc             Immediately before a GC operation
+
+When a program hook script is invoked, there is no context, so no arguments are
+passed to the script.
+
+
+Task Hooks
+----------
+
+These scripts are triggered by task-specific events.  The supported task hooks
+are:
+
+    on-task-add       Immediately prior to committing an added task
+    on-task-modify    Immediately prior to committing a modified task
+    on-task-complete  Immediately prior to committing a completed task
+    on-task-delete    Immediately prior to committing a deleted task
+
+When a task hook script is invoked, the context is a specific task, and so the
+task uuid is passed as the only argument.
+
+
+User Defined Attribute
+----------------------
+
+It is possible to create a user-defined attribute with a UDA extension.  These
+extensions must provide:
+
+    - Data type (string, date, duration, integer, real, custom)
+    - Custom types must implement a compare function for sorting
+    - Default format rendering
+    - Allowed value checking
+
+
+Command
+-------
+
+It is possible to implement a command using an extension.  These extensions must
+provide:
+
+    - BNF command syntax
+    - Declaration as read-only or write command, which allows taskwarrior to
+      allow this command when the database is read-only
+    - Declaration of whether the command displays ID values, which instructs
+      taskwarrior to run a GC beforehan
+
+
+Format
+------
+
+A format extension is one that provides custom rendering for an attribute.
+These extensions must provide:
+
+    - Must implement a format function.
+
+
+DOM
+---
+
+DOM extensions provide a DOM address and can be called to respond to that name.
+These extensions must provide:
+
+    - DOM name
+    - Evaluation function
+
+---
 
diff --git a/scripts/extensions/command.lua b/scripts/extensions/command.lua
new file mode 100644
index 000000000..8a29d995b
--- /dev/null
+++ b/scripts/extensions/command.lua
@@ -0,0 +1,45 @@
+-- Command Extension.
+-- Implementing 'random'.
+
+-- Arguments: None
+-- Returns:   An 8-element list of installation details.  Only called once, on
+--            installation of the extension.
+function install ()
+  return 'command',                        -- Type
+         'random',                         -- Name
+         1.0,                              -- Version
+         'Displays a random pending task', -- Description
+         'Paul Beckingham',                -- Author
+         'paul@beckingham.net',            -- Contact
+         'GPLv2',                          -- License
+         '© 2011, Göteborg Bit Factory'    -- Copyright
+end
+
+-- Arguments: None
+-- Returns:   Valid Taskwarrior BNF, minimally defining a production rule that
+--            has the same name as the command itself
+function syntax ()
+  return 'random ::= "random" ;'
+end
+
+-- Arguments: None
+-- Returns:   1 --> command does not modify data
+--            0 --> command modifies data
+function read_only ()
+  return 1
+end
+
+-- Arguments: None
+-- Returns:   1 --> command displays task ID
+--            0 --> no ID dispalyed
+function display_id ()
+  return 1
+end
+
+-- Arguments: None
+-- Returns:   1, 'error' --> command failed
+--            0, nil     --> success
+function execute ()
+  return 1, 'Not implemented'
+end
+
diff --git a/scripts/extensions/dom.lua b/scripts/extensions/dom.lua
new file mode 100644
index 000000000..a35793e15
--- /dev/null
+++ b/scripts/extensions/dom.lua
@@ -0,0 +1,24 @@
+-- DOM Extension.
+-- Implementing 'system.load.average'.
+
+-- Arguments: None
+-- Returns:   An 8-element list of installation details.  Only called once, on
+--            installation of the extension.
+function install ()
+  return 'dom',                            -- Type
+         'system.load.average',            -- Name
+         1.0,                              -- Version
+         'Provides access to system load', -- Description
+         'Paul Beckingham',                -- Author
+         'paul@beckingham.net',            -- Contact
+         'GPLv2',                          -- License
+         '© 2011, Göteborg Bit Factory'    -- Copyright
+end
+
+-- Arguments: The DOM reference to evaluate
+-- Returns:   The value from the DOM lookup
+-- Note:      'name' may include '*' wildcards
+function lookup (name)
+  return 1.23 -- Fake load average
+end
+
diff --git a/scripts/extensions/format.lua b/scripts/extensions/format.lua
new file mode 100644
index 000000000..d69cd5fbd
--- /dev/null
+++ b/scripts/extensions/format.lua
@@ -0,0 +1,23 @@
+-- Format Extension.
+-- Implementing 'uuid.short'
+
+-- Arguments: None
+-- Returns:   An 8-element list of installation details.  Only called once, on
+--            installation of the extension.
+function install ()
+  return 'format',                         -- Type
+         'uuid.short',                     -- Name
+         1.0,                              -- Version
+         'Provides short formatted UUIDs', -- Description
+         'Paul Beckingham',                -- Author
+         'paul@beckingham.net',            -- Contact
+         'GPLv2',                          -- License
+         '© 2011, Göteborg Bit Factory'    -- Copyright
+end
+
+-- Argument: Value to be formatted
+-- Returns:  Formatted value
+function format (value)
+  return string.sub (value, 0, 8)
+end
+
diff --git a/scripts/extensions/priority.lua b/scripts/extensions/priority.lua
deleted file mode 100644
index 6a4120a6e..000000000
--- a/scripts/extensions/priority.lua
+++ /dev/null
@@ -1,42 +0,0 @@
---------------------------------------------------------------------------------
--- taskwarrior - a command line task list manager.
---
--- Copyright 2006 - 2011, Paul Beckingham, Federico Hernandez.
--- All rights reserved.
---
--- This program is free software; you can redistribute it and/or modify it under
--- the terms of the GNU General Public License as published by the Free Software
--- Foundation; either version 2 of the License, or (at your option) any later
--- version.
---
--- This program is distributed in the hope that it will be useful, but WITHOUT
--- ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
--- FOR A PARTICULAR PURPOSE.  See the GNU General Public License for more
--- details.
---
--- You should have received a copy of the GNU General Public License along with
--- this program; if not, write to the
---
---     Free Software Foundation, Inc.,
---     51 Franklin Street, Fifth Floor,
---     Boston, MA
---     02110-1301
---     USA
---
---------------------------------------------------------------------------------
-
--- Extension to implement 'priority' as a user-defined attribute.
-
--- Required for all extensions.
-function install ()
-  return 'Priority',                       -- Name
-         1.0,                              -- Version
-         'uda',                            -- Type
-         'Implements priority attribute',  -- Description
-         'Paul Beckingham',                -- Author
-         'paul@beckingham.net',            -- Contact
-         '© 2011, Göteborg Bit Factory'    -- Copyright
-end
-
--- TODO Required for UDA extensions.
-
diff --git a/scripts/extensions/program_hook.lua b/scripts/extensions/program_hook.lua
new file mode 100644
index 000000000..6670f68bc
--- /dev/null
+++ b/scripts/extensions/program_hook.lua
@@ -0,0 +1,31 @@
+-- Program Hook Extension.
+-- Implementing goodbye message.
+
+-- Arguments: None
+-- Returns:   An 8-element list of installation details.  Only called once, on
+--            installation of the extension.
+function install ()
+  return 'program',                        -- Type
+         'goodbye',                        -- Name
+         1.0,                              -- Version
+         'Simply says goodbye',            -- Description
+         'Paul Beckingham',                -- Author
+         'paul@beckingham.net',            -- Contact
+         'GPLv2',                          -- License
+         '© 2011, Göteborg Bit Factory'    -- Copyright
+end
+
+-- Arguments: None
+-- Returns:   String identifying valid program hook
+function hook ()
+  return 'on-exit'
+end
+
+-- Arguments: None
+-- Returns:   1, 'error' --> failure
+--            0, nil     --> success
+function goodbye ()
+  print ('Goodbye.')
+  return 0, nil
+end
+
diff --git a/scripts/extensions/task_hook.lua b/scripts/extensions/task_hook.lua
new file mode 100644
index 000000000..71ff31579
--- /dev/null
+++ b/scripts/extensions/task_hook.lua
@@ -0,0 +1,36 @@
+-- Task Hook Extension.
+-- Implementing encouragement message.
+
+-- Arguments: None
+-- Returns:   An 8-element list of installation details.  Only called once, on
+--            installation of the extension.
+function install ()
+  return 'task',                           -- Type
+         'encourage',                      -- Name
+         1.0,                              -- Version
+         'Positive feedback',              -- Description
+         'Paul Beckingham',                -- Author
+         'paul@beckingham.net',            -- Contact
+         'GPLv2',                          -- License
+         '© 2011, Göteborg Bit Factory'    -- Copyright
+end
+
+-- Arguments: None
+-- Returns:   String identifying valid program hook
+function hook ()
+  return 'on-task-complete'
+end
+
+-- Arguments: None
+-- Returns:   1, 'error' --> failure
+--            0, nil     --> success
+function encourage ()
+  -- Only provide encouragement if the verbosity settings allow it.
+  verbosity = task_get ('rc.verbose')
+  if string.find (verbosity, 'encourage') ~= nil
+  then
+    print ('Good work.')
+  end
+  return 0, nil
+end
+
diff --git a/scripts/extensions/uda.lua b/scripts/extensions/uda.lua
new file mode 100644
index 000000000..8ef19b0e4
--- /dev/null
+++ b/scripts/extensions/uda.lua
@@ -0,0 +1,61 @@
+-- User Defined Attribute Extension.
+-- Implementing 'priority'.
+
+-- Arguments: None
+-- Returns:   An 8-element list of installation details.  Only called once, on
+--            installation of the extension.
+function install ()
+  return 'uda',                            -- Type
+         'priority',                       -- Name
+         1.0,                              -- Version
+         'Implements priority attribute',  -- Description
+         'Paul Beckingham',                -- Author
+         'paul@beckingham.net',            -- Contact
+         'GPLv2',                          -- License
+         '© 2011, Göteborg Bit Factory'    -- Copyright
+end
+
+
+-- Arguments: None
+-- Returns:   Data type
+function type ()
+  return 'custom'
+end
+
+-- Arguments: proposed value
+-- Returns:   1 --> allowed
+--            0 --> disallowed
+function allowed (value)
+  if value == 'H' ||
+     value == 'M' ||
+     value == 'L' ||
+     value == '' then
+    return 1
+  end
+
+  return 0
+end
+
+-- Arguments: left and right values to compare
+-- Returns:   1 --> left < right
+--            0 --> left >= right
+function compare (left, right)
+  if left == 'M' && right == 'H' then
+    return 1
+  elseif left == 'L' && (right == 'H' || right == 'M') then
+    return 1
+  elseif left == '' then
+    return 1
+  end
+
+  return 0
+end
+
+-- Arguments: Raw data
+-- Returns:   Formatted data
+-- Note:      Shown here is a pass-through format, doing no formatting.  This is
+--            the default behavior if the format function is not implemented.
+function format (value)
+  return value
+end
+
diff --git a/src/API.cpp b/src/API.cpp
index e18a8226d..6772d93ba 100644
--- a/src/API.cpp
+++ b/src/API.cpp
@@ -53,7 +53,6 @@
 #include <API.h>
 
 extern Context context;
-Task* the_task = NULL;
 
 #ifdef HAVE_LIBLUA
 
@@ -122,6 +121,7 @@ static int api_task_set (lua_State* L)
   catch (...)
   {
     // TODO Error!
+    lua_pushstring (L, "");
   }
 }
 
@@ -223,16 +223,10 @@ bool API::callTaskHook (
   // Prepare args.
   lua_pushnumber (L, current.id);
 
-  // Expose the task.
-  the_task = &current;
-
   // Make call.
   if (lua_pcall (L, 1, 2, 0) != 0)
     throw std::string ("Error calling '") + function + "' - " + lua_tostring (L, -1) + ".";
 
-  // Hide the task.
-  the_task = NULL;
-
   // Call successful - get return values.
   if (!lua_isnumber (L, -2))
     throw std::string ("Error: '") + function + "' did not return a success indicator.";