T lymphocytes recognize specific antigens through interaction of the T cell receptor (TCR) with short peptides presented by major histocompatibility complex (MHC) class I or II molecules. For initial activation and clonal expansion, naïve T cells are dependent on professional antigen-presenting cells (APCs) that provide additional co-stimulatory signals. TCR activation in the absence of co-stimulation can result in unresponsiveness and clonal anergy. To bypass immunization, different approaches for the derivation of cytotoxic effector cells with grafted recognition specificity have been developed. Chimeric antigen receptors have been constructed that consist of binding domains derived from natural ligands or antibodies specific for cell-surface antigens, genetically fused to effector molecules such as the TCR alpha and beta chains, or components of the TCR-associated CD3 complex. Upon antigen binding, such chimeric receptors link to endogenous signaling pathways in the effector cell and generate activating signals similar to those initiated by the TCR complex. Since the first reports on chimeric antigen receptors, this concept has steadily been refined and the molecular design of chimeric receptors has been optimized. Aided by advances in recombinant antibody technology, chimeric antigen receptors targeted to a wide variety of antigens on the surface of cancer cells and of cells infected by human immunodeficiency virus (HIV) have been generated. In initial clinical studies, infusion of such cells into patients proved to be safe and transient therapeutic effects have been observed.