Here's the stats page on wordpress.org: http://wordpress.org/about/stats/ http://wordpress.org/about/stats/ In short, depending on the purpose & features of yo

Here's the stats page on wordpress.org: http://wordpress.org/about/stats/ http://wordpress.org/about/stats/ In short, depending on the purpose & features of your plugin, you may want to consider testing on older versions as well. With themes, it shouldn't be too big of an issue to ensure compatibility for a couple of versions back, at least. Edit: Probably not entirely accurate, see this tweet by a core dev: https://twitter.com/nacin/status/456516848763146240 https://twitter.com/nacin/status/456516848763146240