C2 domains are a ubiquitous structural module and many of them function in Ca2+ -dependent membrane binding and thereby serve as Ca2+ effectors for divergent Ca2+ -mediated cellular processes. Extensive structural, biochemical, biophysical, and cellular studies of C2 domains and host proteins in the past decade have shown that due to their structural diversity C2 domains have disparate Ca2+ sensitivity, lipid selectivity and membrane binding mechanisms. This review summarizes the basic structural and functional properties of C2 domains as well as recent findings on Ca2+ and membrane binding, lipid selectivity, and subcellular localization of C2 domains and their host proteins.