Clinical trials of tumor-antigen-specific immunization have clearly shown that immune-mediated tumor rejection requires more than simple T cell-target cell interactions. In vivo generation of tumor-specific T cells is one of a series of steps necessary for the induction of clinically relevant immune responses. In recent years, high-throughput functional genomics exposed the complexity of tumor immune biology, which underlies the kaleidoscopic array of variables associated with cancer instability and immunogenetic variability in humans. In the quest to understand immune rejection, hypothesis-driven approaches have failed to take into account the intricacy of human pathology by relying mostly on hypotheses derived from experimental models rather than direct clinical observation. Future investigations should reframe scientific thinking when applied to humans, utilizing descriptive tools to generate novel hypotheses relevant to human disease.