The immune system has the ability to recognize and attack tumor cells based on the expression or over-expression of specific antigens, but immune cells often express receptors that poorly recognize tumor antigens. However, recombinant DNA techniques combined with knowledge of immune signaling has provided a means to design powerful new receptors, called chimeric antigen receptors or CARs, that can recognize any target molecule and activate a variety of cell effector functions just at the site where antigen is present. One of the primary challenges using CAR based effector cells is to achieve good efficacy with limited toxicity. There are many different receptor designs that produce efficacious CAR cells, so there will be multiple paths that lead to success.