Fields are those parts of the embryo in which the processes of development of complex structure appropriate to those parts are controlled and coordinated in a spatially ordered, temporally synchronized, and epimorphically hierarchical manner. Disturbances of field development may be corrected or may lead to anomalies of abnormal or incomplete differentiation. Powerful evidence for the existence of developmental fields comes from the observation of identical malformations resulting from two or more different causes; this identifies groups of embryonic structures that respond as a single developmental unit. Early action of a dysmorphogenetic cause in a field may lead to an extensive defect (e.g., cyclopic holoprosencephaly), later action to a lesser defect (e.g., single upper central incisor as mildest expression of autosomal-dominant holoprosencephaly). Disturbances of "long-distance" inductive relationships between primordia may simulate the presence of a malformation syndrome (e.g., renal and limb anomalies as disturbance of the acrorenal developmental field). The biology of developmental fields is complex but accessible through knowledge of comparative anatomy and embryology, phylogeny, experimental embryology, developmental genetics, and teratology. It challenges the clinician to a deeper understanding of development in order to give better care to the malformed and their families.