A substantial gain in the performance of polymer solar cells can be realized with multi-junction solar cells in which two or more absorber layers reduce the thermalization and transmission losses. This chapter explains the operating principles, optimization, and characterization of multi-junction polymer solar cells. The focus is on materials for creating efficient absorber layers and intermediate contacts. The chapter provides a comprehensive overview of the achievements that have pushed the power conversion efficiency of these devices to well over 10%, and addresses the progress in processing of large area tandem polymer solar cells.