Reactive sputter deposition is widely used for growing technologically important ceramic films, including high melting point phases near room temperature, metastable phases, and nanoscale layered structures with controlled interfaces. Film properties are governed by kinetic processes at each electrode and in the gas phase. A knowledge of the reacting species and reaction paths is essential for reproducible growth of desired ceramic phases and structures. Obtaining this knowledge is the first critical step in developing transferrable processes. In this article, we briefly describe reactive sputter deposition of oxides and nitrides from metal and ceramic targets, and identify important chemical features of the process; Production of activated gas species by plasma volume collisions between ground state reactive gas molecules and rare gas atoms in low-lying metastable energy states is discussed. We then review mass and optical spectrometric methods for real-time monitoring of nonelectronic species in the discharge, and using many examples, show how the information obtained from in situ diagnostics gives insight into the chemistry of ceramic film growth.