This chapter summarizes the scientific basis for and the current status of seasonal-to-interannual prediction with particular emphasis on the role of the tropical oceans. The first part of the chapter focuses on oceanic sources of predictability in the tropical Pacific, Atlantic, and Indian Oceans. Seasonal-to-interannual predictability issues in the Northern Hemisphere extratropics are also discussed. Mechanisms that limit predictability, particularly for ENSO, are highlighted. The second part of the chapter describes the forecast quality and procedures in practice today. Finally, the concluding remarks identify some outstanding challenges.