Longins are the only R-SNAREs that are common to all eukaryotes and are characterized by a conserved N-terminal domain with a profilin-like fold called a longin domain (LD). These domains seem to be essential for regulating membrane trafficking and they mediate unexpected biochemical functions via a range of protein-protein and intramolecular binding specificities. In addition to the longins, proteins involved in the regulation of intracellular trafficking, such as subunits of the adaptor and transport protein particle complexes, also have LD-like folds. The functions and cellular localization of longins are regulated at several levels and the longin prototypes TI-VAMP, Sec22 and Ykt6 show different distributions among eukaryotes, reflecting their modular and functional diversity. In mammals, TI-VAMP and Ykt6 are crucial for neuronal function, and defects in longin structure or function might underlie some human neurological pathologies.