The onset and regulation of a specific immune response results from communication between T cells and antigen-presenting cells (APCs), which form molecular interactions at the site of cell–cell contact — and this is known as the immunological synapse. Initially, the immunological synapse was viewed as a stereotypical adhesion and signalling device with a defined molecular structure and signalling processes. However, as we discuss here, T-cell–APC interactions comprise a diverse range of contact modes and distinct molecular arrangements. These diverse interaction modes might define a molecular code, in which the differences in timing, spacing and molecular composition of the signalling platform determine the outcome of T-cell–APC interactions.