Because of the high relapse rate of resected gastrointestinal malignancies and the modest responses of metastatic disease to currently available therapies, biologic agents that harness host-tumor immunologic interactions have received increased attention. Based on promising preclinical data, current clinical trials in cellular and biologic therapies are evaluating the safety and efficacy of passive immunotherapy with tumor-reactive lymphocytes activated ex vivo and active immunotherapy with peptide, viral vector, and cellular vaccines. This review will describe the background, rationale, and experimental approach of these clinical trials. Although equally promising, antibodies, gene therapies, and antiangiogenic strategies will not be discussed.