FOREWORDWhereas the discussion of epitope-antigen structure relating to B cells would not be very different from a similar consideration a decade ago, the nature of T-cell recognition and activation by "antigen" encompasses a completely altered area of concern. Within the past 10 years, concepts of restriction to major histocompatibility-complex MHC molecules and in volvement of T cells in idiotypic interactions have revolutionized the view of the "antigen" that activates the T cells.In this article we use a special definition for the term antigen structure, which includes all elements shown to be recognized by the diversity ofT-cell subpopulations: epitopes (from the nominal antigen), idiotopes (on secreted Ig or on other cell surfaces), and at least two types of MHC determinants, which will be defi ned later.We do not undertake a review of the basis and evidence for MHC restric tion, or T-cell idiotypic interactions: earlier excellent reviews on the former (57,139,148,186) and latter (47,82,83,175) exist, together with several in the present volume. Our focus is confined to current information describ ing the complexity of structures involved in T-cell activation.