Formins are a large family of multi-domain polypeptides that form homodimers. The highly conserved formin homology 2 (FH2) domain and its neighboring formin homology 1 (FH1) domain, which are surrounded by regulatory domains, cooperate in rapidly assembling profilin-actin into long filaments while remaining continuously associated with the fast-growing barbed end. Recent biochemical, biophysical, theoretical and structural studies have concluded that diverse formins are mechanistically similar, but that the rates of various assembly states differ quantitatively, and have shed light on the mechanism of formin auto-regulation and activation by Rho GTPases.