Mismatch repair proteins are a highly diverse group of proteins that interact with numerous DNA structures during DNA repair and replication. Here we review data for the role of Msh4, Msh5, Mlh1, Mlh3 and Exo1 in crossing over. Based on the paradigm of interactions developed from studies of mismatch repair, we propose models for the mechanism of crossover implementation by Msh4/Msh5 and Mlh1/Mlh3.