Perhaps half of all patients with multiple abnormalities have known, recognized syndromes. The other half represent unknown entities that need to be further delineated. The significance of syndrome delineation cannot be overestimated. As an unknown syndrome becomes delineated, its phenotypic spectrum, its natural history, and its risk of recurrence become known, allowing for better patient care and family counseling. The process of syndrome delineation is discussed in terms of unknown genesis syndromes of the previsionally-unique and recurrent-pattern types, and known-genesis syndromes of the pedigree, chromosomal, biochemical-defect, and environmentally-induced types. Several special syndrome categories are defined, including the chance syndrome, variant additive syndrome, association syndrome, exceptional chromosomal syndrome, and exceptional monogenic syndrome. Finally, a population definition of a syndrome is developed.