Members of the structural maintenance of chromosomes (SMC) family share a characteristic design and configuration of protein domains that provides the molecular basis for the various functions of this family in chromosome dynamics. SMC proteins have a role in chromosome condensation, sister-chromatid cohesion, DNA repair and recombination, and gene dosage compensation, and they function in somatic and meiotic cells. As more is learned about how their unique design affects their function, a picture of a dynamic and varied protein family is emerging.