Cytokine-induced killer (CIK) cells form under certain stimulation conditions in cultures of peripheral blood mononuclear cells (PBMCs). They are a heterogeneous immune cell population and contain a high percentage of cells with a mixed T-NK phenotype (CD3+CD56+). The ready availability of a lymphocyte source, together with the high proliferative rate and potent anti-tumor activity of CIK cells, has allowed their use as immunotherapy in a wide variety of neoplasms. Cytotoxicity mediated by CD3+CD56+ T cells depends on the major histocompatibility antigen (MHC)-independent recognition of tumor cells and the activation of signaling pathways through the natural killer group 2 member D (NKG2D) cell-surface receptor. Clinical trials have demonstrated the feasibility and efficacy of CIK cell immunotherapy even in advanced stage cancer patients or those that have not responded to first-line treatment. This review summarizes biological and technical aspects of CIK cells, as well as past and current clinical trials and future trends in this form of immunotherapy.