BAR (bin, amphiphysin and Rvs161/167) domains are a unique class of dimerization domains, whose dimerization interface is edged by a membrane-binding surface. In its dimeric form, the membrane-binding interface is concave, and this gives the ability to bind better to curved membranes, i.e. to sense membrane curvature. When present at higher concentrations, the domain can stabilize membrane curvature, generating lipid tubules. This domain is found in many contexts in a wide variety of proteins, where the dimerization and membrane-binding function of this domain is likely to have a profound effect on protein activity. If these proteins function as predicted, then there will be membrane subdomains based on curvature, and thus there is an additional layer of compartmentalization on membranes. These and other possible functions of the BAR domain are discussed.