Visualization has become an accepted means for data exploration and analysis. Although interaction is an important component of visualization approaches, much of the visualization literature tends to pay more attention to aspects of the graphical representation than to interaction. The goal of this work is to strengthen the interaction side of visualization. We establish a unified view on interaction in visualization. This unified view covers four key aspects: the data, the tasks, the technology, and the human. Addressing challenges and questions related to these aspects, we introduce concepts and techniques for interaction in the context of visualization. We deal with interaction for various types of data, including graph data and movement data. We consider interaction approaches for exploration, navigation, visual comparison, and visual editing. Approaches including tangible and proxemic interaction illustrate modern forms of interaction in visualization. While interactive lenses will play an important role, we also include automatic methods to assist the interactive part. In addition to presenting solutions for individual problems, a major contribution of this work is the overarching discussion of interaction in visualization as a whole. This also includes the formulation of research topics for future work with a focus on interaction.