Interactivity is a fundamental property of items in the real world. We expect to be able to turn things on, move them around, change locations, or at the very least shift our viewpoint so as to see them in a different way. We color objects, resize and reshape them, and interact with them in many ways. Exploration via interactivity is a basic children’s activity, leading to understanding. It is a key way of learning we encourage in children and is no less valuable for adults. An old Chinese adage, often attributed to Confucius, states “I hear and I forget. I see and I remember. I do and I understand.” That is the basic premise of interactivity. It leverages the way we achieve understanding in the physical world, building on the visual (“I see and I remember”) and enforcing it with actions that allow us to manipulate displays and so improve our knowledge of the data that are being visualized (“I do and I understand”). Interactive data visualizations allow users to see pop-ups and tool tips; rotate, pan, and zoom in on plots; selectively show details in some areas while maintaining an overall view of the context; drill down into subareas of the data space; manipulate parameters such as degree of smoothing; select parts of one view and see corresponding items highlighted in another view; and dynamically filter data with controls or via direct manipulation.