The BAR (Bin/Amphiphysin/Rvs) domain is a conserved structural motif found in amphiphysins, endophilins, and nadrins. This study reports the structure of the Drosophila amphiphysin BAR domain, which forms a crescent-shaped dimer that preferentially binds to highly curved, negatively charged membranes. The N-terminal amphipathic helix and BAR domain (N-BAR) of amphiphysin can drive membrane curvature in vitro and in vivo. The structure is similar to arfaptin2, which also binds and tubulates membranes. The BAR domain is a dimerization, membrane-binding, and curvature-sensing module. Amphiphysins are brain-enriched proteins that bind to clathrin, AP2, dynamin, and synaptojanin, and are involved in synaptic vesicle endocytosis. In Drosophila, amphiphysin is associated with muscle T-tubule networks and areas of membrane remodeling. The BAR domain is conserved across amphiphysins from yeast to humans and is also found in endophilins and nadrins. The BAR domain binds and tubulates liposomes in vitro, and the N-BAR domain is essential for the formation and stability of the muscle T-tubule network in vivo. The structure of the Drosophila amphiphysin BAR domain was determined by X-ray crystallography. It is an elongated banana-shaped dimer with a six-helix bundle. The N-terminal amphipathic helix and BAR domain form an N-BAR, which is important for membrane curvature. The concave surface of the dimer has positively charged patches, suggesting it interacts with phospholipid membranes. The BAR domain can sense membrane curvature, as evidenced by its preference for curved membranes. The BAR domain is also involved in recruiting clathrin to membranes and polymerizing it into invaginated lattices. Arfaptin2 also contains a BAR domain, which is structurally similar to the amphiphysin BAR domain. Arfaptin2 binds to liver liposomes more effectively than brain or synthetic liposomes containing PtdIns(4,5)P₂. The BAR domain of arfaptin2 is involved in membrane tubulation and is essential for the formation of tubular structures in cells. BAR domains are found in many protein families, including sorting nexins, centaurins, and oligophrenins. These domains are involved in membrane curvature sensing and are essential for the spatial and temporal compartmentalization of proteins to specific membrane domains. The BAR domain is a dimerization, membrane-binding, and curvature-sensing module that is found across genomes and in many different protein contexts.The BAR (Bin/Amphiphysin/Rvs) domain is a conserved structural motif found in amphiphysins, endophilins, and nadrins. This study reports the structure of the Drosophila amphiphysin BAR domain, which forms a crescent-shaped dimer that preferentially binds to highly curved, negatively charged membranes. The N-terminal amphipathic helix and BAR domain (N-BAR) of amphiphysin can drive membrane curvature in vitro and in vivo. The structure is similar to arfaptin2, which also binds and tubulates membranes. The BAR domain is a dimerization, membrane-binding, and curvature-sensing module. Amphiphysins are brain-enriched proteins that bind to clathrin, AP2, dynamin, and synaptojanin, and are involved in synaptic vesicle endocytosis. In Drosophila, amphiphysin is associated with muscle T-tubule networks and areas of membrane remodeling. The BAR domain is conserved across amphiphysins from yeast to humans and is also found in endophilins and nadrins. The BAR domain binds and tubulates liposomes in vitro, and the N-BAR domain is essential for the formation and stability of the muscle T-tubule network in vivo. The structure of the Drosophila amphiphysin BAR domain was determined by X-ray crystallography. It is an elongated banana-shaped dimer with a six-helix bundle. The N-terminal amphipathic helix and BAR domain form an N-BAR, which is important for membrane curvature. The concave surface of the dimer has positively charged patches, suggesting it interacts with phospholipid membranes. The BAR domain can sense membrane curvature, as evidenced by its preference for curved membranes. The BAR domain is also involved in recruiting clathrin to membranes and polymerizing it into invaginated lattices. Arfaptin2 also contains a BAR domain, which is structurally similar to the amphiphysin BAR domain. Arfaptin2 binds to liver liposomes more effectively than brain or synthetic liposomes containing PtdIns(4,5)P₂. The BAR domain of arfaptin2 is involved in membrane tubulation and is essential for the formation of tubular structures in cells. BAR domains are found in many protein families, including sorting nexins, centaurins, and oligophrenins. These domains are involved in membrane curvature sensing and are essential for the spatial and temporal compartmentalization of proteins to specific membrane domains. The BAR domain is a dimerization, membrane-binding, and curvature-sensing module that is found across genomes and in many different protein contexts.