The correct differential diagnosis of an abnormal head shape in an infant or a child is vital to the management of this common condition. Establishing the presence of craniosynostosis, which warrants surgical correction, versus non-synostotic causes of head deformity, which do not, is not always straightforward. This paper deals with three groups of abnormal head shape that may cause diagnostic confusion: the spectrum of metopic synostosis; the dolichocephaly of prematurity versus sagittal synostosis; and the differential diagnosis of plagiocephaly. Special emphasis has been placed on the problem of posterior plagiocephaly, in the light of recent evidence demonstrating that lambdoid synostosis has been overdiagnosed. Metopic synostosis presents as a wide spectrum of severity. Although only severe forms of the disorder are corrected surgically, all cases should be monitored for evidence of developmental problems. The dolichocephalic head shape of preterm infants is non-synostotic in origin and is managed nonsurgically. The scaphocephalic head shape resulting from sagittal synostosis requires surgical intervention for correction. Posterior plagiocephaly may be due to unilambdoid synostosis or positional molding, which have very different clinical and imaging features. True lambdoid synostosis is rare. Most cases of posterior plagiocephaly are due to positional molding, which can usually be managed nonsurgically. Regardless of the suture(s) involved, all children with confirmed craniosynostosis should be monitored for increased intracranial pressure and developmental problems.