Fibular aplasia and/or hypoplasia is documented as a developmental field defect and the extent of the fibular developmental field is delineated. The term fibular a/hypoplasia denotes the clinical spectrum of fibular deficiency in different patients and also implies that aplasia can be present in one limb and hypoplasia in the other. Causal heterogeneity of fibular a/hypoplasia is demonstrated, thereby defining it as a developmental field defect. Most cases of fibular a/hypoplasia are isolated, sporadic events. An autosomal dominant form of isolated fibular a/hypoplasia with ankle joint anomaly is reviewed. Fibular a/hypoplasia may be part of more complex sporadic dysostoses; sporadic syndromes, an aneuploidy syndrome; several autosomal dominant and autosomal recessive conditions. Fibular a/hypoplasia is also postulated to occur as a result of disruption or teratogenic insult; in animals, fibular development can be disturbed by radiation, busulfan, and retinoic acid. Clinical data allow evaluation of the extent of the fibular developmental field of the lower limb. This appears to include the pubic portion of the pelvis, proximal femur (distal half being apparent tibial developmental territory), patella, anterior cruciate ligament, and lateral and/or axial foot rays (but “never” the hallux and almost never associated with polydactyly). The rare cases of fibuloulnar dimelia allow confirmation of the well known homology of mesomelic limb segments responsible for concordant ulnar and fibular (and radial and tibial) defect, if both upper and lower limbs are involved in a given condition. Because fibular a/hypoplasia is the commonest of the mesomelic paraxial hemimelias, is usually nonsyndromal, and in most cases is apparently nongenetic (ie, with negligible recurrence risk), we propose that in humans, as in several other tetrapods, the fibula is undergoing regressive evolution and hence is developmentally especially labile.