SR proteins are a family of RNA binding proteins that contain a signature RS domain enriched with serine/arginine repeats. The RS domain is also found in many other proteins, which are collectively referred to as SR-related proteins. Several prototypical SR proteins are essential splicing factors, but the majority of RS domain-containing factors are characterized by their ability to alter splice site selection in vitro or in transfected cells. SR proteins and SR-related proteins are generally believed to modulate splice site selection via RNA recognition motif-mediated binding to exonic splicing enhancers and RS domain-mediated protein-protein and protein-RNA interactions during spliceosome assembly. However, the biological function of individual RS domain-containing splicing regulators is complex because of redundant as well as competitive functions, context-dependent effects and regulation by cotranscriptional and post-translational events. This chapter will focus on our current mechanistic understanding of alternative splicing regulation by SR proteins and SR-related proteins and will discuss some of the questions that remain to be addressed in future research.