Neurexins are neuronal cell-surface proteins with up to thousands of isoforms. These isoforms are generated by alternative splicing of transcripts from six promoters in three genes. The structure of neurexins resembles cell-surface receptors with a modular architecture suggestive of a sequential assembly during evolution. Neurexins probably perform multiple functions in the brain. They participate in intercellular junctions in which beta-neurexins tightly bind to a second class of neuronal cell-surface receptors called neuroligins. Intracellularly, the neurexin/neuroligin junction is bound by CASK on the neurexin side and PSD95 on the neuroligin side. CASK and PSD95 are homologous membrane-associated guanylate kinases that bind to the neurexin/neuroligin junction via PDZ domains, creating an asymmetric junction (neurexin/neuroligin) with similar intracellular binding partners. In addition to a function as cell-adhesion molecules, neurexins may also serve as a signalling receptor, because a class of ligands for alpha-neurexins called neurexophilins is similar to peptide hormones. Finally, at least one neurexin isoform, neurexin Ialpha, represents a high-affinity receptor for alpha-latrotoxin, which is a potent excitatory neurotoxin. Thus, neurexins constitute a large family of neuronal receptors that may be involved in multiple interactive functions between neurons.