Desmosomes are intercellular adhesive junctions that impart strength to vertebrate tissues. Their dense, ordered intercellular attachments are formed by desmogleins (Dsgs) and desmocollins (Dscs), but the nature of trans-cellular interactions between these specialized cadherins is unclear. Here, using solution biophysics and coated-bead aggregation experiments, we demonstrate family-wise heterophilic specificity: All Dsgs form adhesive dimers with all Dscs, with affinities characteristic of each Dsg:Dsc pair. Crystal structures of ectodomains from Dsg2 and Dsg3 and from Dsc1 and Dsc2 show binding through a strand-swap mechanism similar to that of homophilic classical cadherins. However, conserved charged amino acids inhibit Dsg:Dsg and Dsc:Dsc interactions by same-charge repulsion and promote heterophilic Dsg:Dsc interactions through oppositecharge attraction. These findings show that Dsg:Dsc heterodimers represent the fundamental adhesive unit of desmosomes and provide a structural framework for understanding desmosome assembly. Dysfunction of desmosomes in inherited and acquired human diseases as well as in mouse genetic ablation studies causes characteristic defects in heart muscle and skin (3-5), demonstrating their importance in tissues that undergo mechanical stress. In electron micrographs, the hallmarks of mature desmosomes include a constant intermembrane distance of 280-340 Å, and apparently ordered electron-density in the intercellular space, often with a discrete midline connected by periodic cross-bridges to the cell membranes (6-9). The intercellular attachments of desmosomes are composed of transmembrane proteins from two specialized cadherin subfamilies: desmocollins (Dscs) and desmogleins (Dsgs). The human genome encodes three Dsc (Dsc1-Dsc3) and four Dsg (Dsg1-Dsg4) proteins, which share an overall domain organization comprising four to five extracellular cadherin (EC) domains, a single-pass transmembrane region, and an intracellular domain that binds to intermediate filaments via adaptor proteins desmoplakin and plakoglobin (1). Individual Dsgs and Dscs show differential expression patterns: Dsg2 and Dsc2 are expressed widely in all desmosome-forming tissues (1), whereas other desmosomal cadherins are expressed specifically in stratified epithelia with graded, overlapping patterns (1, 10). Notably, both Dscs and Dsgs appear necessary for adhesion in transfected cells (1,(11)(12)(13), and loss of either in genetic experiments causes loss of normal desmosomal adhesion (5,14,15).Although the ultrastructure of desmosomes is well characterized, a molecular-level understanding of the binding interactions between desmosomal cadherin extracellular regions that assemble these junctions has remained elusive. In particular, whether desmosomal cadherins have homophilic preferences or whether interactions occur between heterophilic pairs has been a matter of dispute (1,(11)(12)(13)(16)(17)(18). Electron tomography studies of native desmosomes (7, 8) have revealed cadherins binding through their EC1 domains...