Recognition of foreign antigens by T lymphocytes is a very important component of vertebrate immunity—vital to the clearance of pathogenic organisms and particular viruses and necessary, indirectly, for the production of high affinity antibodies. T cell recognition is mediated by the systematic scanning of cell surfaces by T cells, which collectively express many antigen receptors. When the appropriate antigenic peptide bound to a molecule of the major histocompatibility complex is found—even in minute quantities—a series of elaborate cell-surface molecule and internal rearrangements take place. The sequence of events and the development of techniques required to observe these events have significantly enhanced our understanding of T cell recognition and may find application in other systems of transient cell:cell interactions as well.