Tumor-infiltrating lymphocytes (TIL) are associated with survival in virtually every human cancer, but the mechanisms by which they confer protective immunity remain incompletely understood. Focusing on ovarian cancer, our group applies genomic and molecular pathology approaches to define the mechanisms by which the human immune system responds to the evolving tumor genome over space and time. We find that optimal anti-tumor immunity involves interactions between T cells and antibody-producing B cells in the tumor microenvironment. We have evidence that T cell clones track tumor clones over space and time and apply selective pressure that leads to reduced tumor clone diversity and progressive loss of immune recognition through several mechanisms. Our findings suggest new strategies to overcome these challenges through T cell engineering and other approaches. Toward this goal, I will discuss our cancer centre's new clinical trials program focused on T cell engineering strategies for gynecological and lymphoid cancers.