T cells are often referred to as the 'guided missiles' of our immune system because of their capacity to traffic to and accumulate at sites of infection or disease, destroy infected or mutated cells with high specificity and sensitivity, initiate systemic immune responses, sterilize infections, and produce long-lasting memory. As a result, they are a common target for a range of cancer immunotherapies. However, the myriad of challenges of expanding large numbers of T cells specific to each patient's unique tumor antigens has led researchers to develop alternative, more scalable approaches. Biomaterial platforms for expansion of antigen-specific T cells offer a path forward towards broadscale translation of personalized immunotherapies by providing "off-the-shelf", yet modular approaches to customize the phenotype, function, and specificity of T cell responses. In this review, we discuss design considerations and progress made in the development of ex vivo and in vivo technologies for activating antigen-specific T cells, including artificial antigen presenting cells, T cell stimulating scaffolds, biomaterials-based vaccines, and artificial lymphoid organs. Ultimate translation of these platforms as a part of cancer immunotherapy regimens hinges on an in-depth understanding of T cell biology and cell-material interactions.