Much excitement of the past five years in the area of T-cell-antigen recognition has centred around the immunological synapse — a complex cellular structure that forms at the interface of a T cell and a cell that expresses the appropriate peptide–MHC complexes. Thanks to new imaging technologies, we are now beginning to understand the role of cell-surface molecules and some of their attendant signalling modules in the context of cell-to-cell communication. Progress has been so rapid that T-cell-antigen recognition might be the first system in which the molecular basis of cell–cell recognition is understood.