This paper focuses on the terminology, classification, pathologic anatomy, embryology, and etiology of common atrioventricular (AV) canal. The designation common AV canal is preferred because it includes both the characteristic septal and leaflet defects; i.e. common AV canal is much more than an AV septal defect, as the cleft in the anterior mitral leaflet indicates. This cleft is indeed a cleft (not a commissure), and the left- sided AV valve is a malformed mitral valve (not a trileaflet non-mitral valve). Common AV canal is classified as complete, partial, transitional, intermediate, balanced, left ventricular type, and right ventricular type. The complete form is subclassified as Rastelli types A, B, and C. The normal embryology of AV canal division is presented in man and in the rat. In humans, the superior and inferior endocardial cushions of the AV canal normally fuse during Streeter's horizon XVII, i.e. 34-36 days of age. An understanding of the spatial geometry of the endocardial cushions of the AV canal relative to the ventricular and atrial septa helps to explain normal and abnormal development and the classification of common AV canal. Normal morphogenesis is a three-step process. An increase in fibroblast cell-surface adhesiveness may well be important in the morphogenesis of common AV canal in Down syndrome. Etiologically, a single gene stochastic model is now thought more likely than the polygenic multifactorial hypothesis. The Down syndrome congenital heart disease gene (or genes) is (are) now thought to be located on chromosome 21 from q22.1 to q22.3. Common AV canal may be non-syndromic or syndromic, the latter including Down syndrome and the heterotaxy syndromes with asplenia and polysplenia. There are statistically highly significant differences between Down and non-Down canals, and between asplenic and polysplenic canals.