The collaboration with domain experts concentrates always on an application domain where the experts work. Usually, they provide the data and directions of research that require visualization support. This chapter presents seven successful cases of such collaborations. The domain varies from biology and medicine to mechanical engineering. There are examples of long time cooperation as well as smaller short-term projects. The description concentrates on the process, output, and especially on the lessons learnt from these cooperations. The scientific work is described to understand the context and goals of the cooperation, but many details can only be found in the references. The reason for this unusual writing is the wish on the one hand to describe various aspects of collaboration with domain experts which is an important part of the foundations of data visualization. On the other hand, the text should not become lengthy and filled with too many details of individual cases that can be found elsewhere.