Cytidine 5'-monophosphosialic acid (CMP-sialic acid) is the activated form of sialic acid which is required for the biosynthesis of sialic acid-containing complex carbohydrates. Its discovery over 30 years ago by the laboratory of Dr Saul Roseman was a landmark in research dealing with the biosynthesis of these compounds. A review is presented of the salient features concerning this molecule: its discovery, chemistry, biosynthesis, subcellular location, enzymatic cleavage, transport and molecular biology. This report does not deal with its utilization by the sialyltransferases.