1. A summary is provided of current classifications of receptors for the adenine nucleosides and nucleotides. 2. The origin of the division of nucleoside (P1) and nucleotide (P2) receptors is discussed as well as the subdivisions of these into A1 and A2, determined by structure activity considerations, and P2x and P2y. 3. Further suggestions have been made recently for identifying A1a and A1b, A2a and A2b subtypes of the P1 receptor family, and for proposing distinctive nucleotide receptors on platelets (P2t) and mast cells (P2z). 4. In addition, an A3 site may exist with properties intermediate between A1 and A2, and a novel P2s receptor on some smooth muscle systems and a P3 site.