Tissue resident memory (TRM) cells permanently reside within barrier tissues where they provide protection against a plethora of invading pathogens and cells of tumorigenic origin. TRM cells represent a unique memory population, sharing a core residency signature across various tissues distinct from that of circulating memory T cells. Tumour-infiltrating lymphocytes (TIL) share aspects of this TRM cell signature, with the presence of TIL-TRM associated with improved survival in cancer patients. It is well established that the circulating memory T cell pool is highly heterogeneous, yet TRM cells have been typically viewed as a relatively homogenous population identified by the canonical CD8+ TRM cell markers CD103 and/or CD69. However, recent findings have challenged this notion, showing that CD8+ TRM cells within a given tissue can have diverse functions, express different transcription factors, and display altered cytokine profiles. Our understanding of the extent of TRM cell diversity, and the implications of this heterogeneity for immune protection is only starting to be uncovered. Furthermore, the factors that control TRM cell heterogeneity are not known. Using viral infection and tumour models, we aim to uncover what factors govern TRM cell diversity within tissues and the tumour microenvironment. Such discoveries will be key in the development of novel immunotherapeutic strategies designed to target TRM cell populations.