When a new major version of WordPress is released, such as WordPress 2.1 or 2.2, you may find that your Themes and Plugins no longer work the way you are used to. It could be that they are completely broken, because something in WordPress that was fundamental to them changed, but it's also possible that you can get them working again by updating some settings, or maybe just a small edit.
This article is part of a series on Migrating Plugins and Themes. It covers changes between WordPress Version 2.1 and Version 2.2, and what you need to do to Plugins and Themes to get them working in Version 2.2.
If the Theme or Plugin you are using was developed by someone else, it is worth checking the Plugin or Theme's home page for compatibility information.
If you are the author of your Theme or Plugin, or if you have modified or customized your Theme, then this article will help you upgrade your Theme/Plugin for 2.2. If your Theme or Plugin is distributed publicly, after getting it working you may want to add it to the Compatibility lists above, so your users will know it is working and which version they should be running.
The main change of importance to Plugins and Themes in Version 2.2 was the introduction of WordPress Widgets as part of the core of WordPress. (Widgets are small pieces of WordPress output, such as category lists, archive lists, and search forms, which you can put into the sidebar(s) of your blog.)
If you want to use Widgets in a Theme that was developed prior to WordPress 2.2, you can find out how to update your theme in this article on Widgetizing Themes.
If you want some output that your Plugin generates to be displayed in a sidebar Widget, you can find out how in this article on Widgetizing Plugins, and you may also find the Widgets API article useful.