The aim of this chapter is to introduce the main topics in the Mediterranean circulation research and to give a general description of the Mediterranean Sea variability. Its scope is to provide information that is important for a comprehensive view of the most relevant issues concerning climate, which is a coupling between atmosphere and sea. The chapter describes data, research results, and key findings that have been reported in the most recent literature about the Mediterranean Sea circulation. The issue about sea level is considered in Chapter 4. The chapter also includes, in its concluding section, a discussion of the main open issues in Mediterranean Sea research.