Oncoimmunotherapy with immune checkpoint inhibitor–targeted antibodies has developed as the most significant advance in the management of cancer in recent years. The concept that the immune system was unsuccessful in protecting humans against the development of cancer has changed over the last decade. Checkpoint molecules are inhibitory (PD-1, PDL-1, CTLA-4, TIM-3, LAG-3, BTLA, and HEVM) and stimulatory (CD27, CD40, OX40, GITR, ICOS, and CD137) co-receptors expressed mostly by T cells, but also by other immune cells including antigen-presenting dendritic cells. The basic function of these inhibitory co-receptors is to negatively regulate T cell activation, which is critical in the maintenance of peripheral self-tolerance. The co-inhibitory receptor ligands for these immune checkpoint molecules are, however, also significantly upregulated in various types of cancers, resulting in evasion of anticancer immunity.