What is “backward compatibility”, and why is it so important to WordPress? I’ll describe a couple of examples from the history of the project that demonstrate the hoops that WordPress has jumped through – or has chosen not to jump through – in the name of back-compat and user trust. I’ll talk about how the project’s approach to backward compatibility underscores the development of nearly all new features in WordPress. And I’ll say a few words about the lessons that plugin and theme developers might want to take from WP core’s approach to back-compat.