Trafficking of protein and lipid cargo through the secretory pathway in eukaryotic cells is mediated by membrane-bound vesicles. Secretory vesicle targeting and fusion require a conserved multi-subunit protein complex termed the exocyst, which interacts with proteins and lipids at sites of polarized exocytosis. The exocyst is a member of the CATCHR family of multi-subunit tethering complexes; these complexes share strong structural homology with <10% sequence identity. Beside a proposed role in specificvesicle tethering, the exocyst is directly involved in regulating SNARE complexes and SNARE-mediated membrane fusion. Moreover, the exocyst complexdirectly interacts with the exocytic Sec1/Munc18 protein. Our studies indicate functional cooperation between the exocyst and Sec1p in regulating SNARE complex assembly and membrane fusion.