This chapter shares our observations, research, and experience with creating interactivity. We explore useful techniques for creating interactive science-oriented online displays, and describe a series of occasions and methods for making exhibits interactive. For each technique, the design issue is described, the methods for addressing the issue are summarized, and there is a discussion of the approach. We explore what kinds of interactivity have proven to work well online, and, perhaps more importantly, what does not work. Generally, technical solutions are prescriptive rather than descriptive, leaving the actual implementation up to the programmers involved in the project.