Changes in our concepts of angiotensin I converting enzyme are reviewed briefly. The actions of this enzyme go beyond liberating angiotensin II from angiotensin I or inactivating bradykinin. Its very wide distribution in the body and its activity in vitro indicate involvement in the metabolism of other biologically active peptides. The recent molecular cloning of the human enzyme confirmed the existence of a hydrophobic C-terminal peptide that forms the short transmembrane domain of this plasma membrane-bound enzyme. The much longer external portion contains two homologous active site domains but probably only one functional active center. Finally, in spite of the great progress made in studying angiotensin converting enzyme, there are many challenging problems waiting to be solved.