Cohesin, a eukaryotic SMC complex, plays a crucial role in mediating sister chromatid cohesion and organizing the genome during interphase. To fulfill these functions, cohesin must act as both a stable guardian of genome integrity, capable of holding sister chromatids together for decades in some cell types, and a dynamic molecular machine that rapidly forms and remodels DNA loops in response to chromatin changes. These processes involve a complex series of interactions between cohesin and its many regulatory subunits, and DNA, the details of which are still under investigation. Here, we describe methods used routinely in our laboratory for studying transient protein–protein interactions and protein–DNA interactions, both in vivo and in vitro, using site-specific cysteine–cysteine crosslinking.