Summary o 1. A review of the classical concepts about the development and partitioning of the truncus-conus and the formation of the membranous ventricular septum of the human heart is made. 2. An ontogenetic theory is proposed to explain congenital truncoconal malformations. This theory is based upon the work of Davis, Streeter, and Kramer, as well as on the study of pathologic specimens from the Instituto Nacional de Cardiologia of Mexico, and from medical literature. 3. Our ontogenetic theory considers truncoconal congenital malformations as a consequence of developmental alterations of the conoventricular flange, the truncoconal ridges, and the primordia of the aortic and pulmonary cusps. 4. Old concepts are re-evaluated and new ones are established in relation to lateral positions, transposition, unequal partitioning at the expense of the aorta or pulmonary artery, common truncus, and valvular defects. 5. Anatomic and embryologic correlations are made in order to permit the pathologic diagnosis of these malformations. 6. The different pictures of isolated and combined trucoconal malformations (which actually exist or may theoretically exist according to the proposed theory) are described. 7. More common congenital truncoconal malformations are interpreted according to this theory: common truncus, Eisenmenger's complex, Fallot's tetralogy, truncus solitarius aorticus, transposition of the great vessels, and the Taussig-Bing complex.