I am an unashamed convert to interactive video. But in my process of conversion I have had to learn some key terms from my new technical catechism. It has been painful but inevitable and in this article I want to offer a quick summary by discussing one key issue — COMPATIBILITY. Will what you want go with what you've got? Should you dump your existing system for a new one? Will you be able to add more later? Will another manufacturing technology render your system obsolete in a few years' time on price, technical or incompatibility grounds? These are the problems facing users and worrying potential users right now — and not all of them have easy (or indeed any) answers.