This chapter looks at the range of ways that online publishers have integrated interactivity into their content, the increasing importance of programming skills for journalists and the tools and techniques. On the communication side, interactivity means allowing users to communicate with the news organisation, with individual journalists and – crucially — each other. A user's name will likely be different every time one piece of code runs. A library is a collection of code that has been made available for other people to use. Just as a real-life library contains lots of books to help people do things, a library in programming makes it easier for people to perform certain tasks. Lists and dictionaries are special types of information that can be enormously useful in programming – but also confusing for those not used to them. The best online journalism exploits the potential of the medium to create the most effective way to communicate information and engage the user.