Activation of immune cells is often achieved via ligand-receptor interactions occurring at a cell-cell junction known as an immunological synapse (IS). Synapse structures, probably best studied in the context of T cell-antigen presenting cell (APC) interactions, are characterized by clustering of cell surface receptors and intracellular signaling components, and in some settings, the formation of microscale or submicron patterns of receptors in the cell-cell interface. To help expand our understanding of how synapses form and function, substrates bearing patterned protein ligands are being developed as simplified models of the APC surface. These new tools allow well-defined signal inputs to be delivered to the T cell in order to ask how the physical organization and composition of APC-derived signals control T cell activation.