Objective: Autosomal recessive primary microcephaly (MCPH) is a rare condition characterized by a reduced cerebral cortex accompanied with intellectual disability. Mutations in 17 genes have been shown to cause this phenotype. Recently, mutations in CIT, encoding CRIK (citron rho-interacting kinase)—a component of the central spindle matrix—were added. We aimed at identifying novel MCPH-associated genes and exploring their functional role in pathogenesis. Methods: Linkage analysis and whole exome sequencing were performed in consanguineous and nonconsanguineous MCPH families to identify disease-causing variants. Functional consequences were investigated by RNA studies and on the cellular level using immunofluorescence and microscopy. Results: We identified homozygous mutations in KIF14 (NM_014875.2;c.263T>A;pLeu88*, c.2480_2482delTTG; p.Val827del, and c.4071G>A;p.Gln1357=) as the likely cause in 3 MCPH families. Furthermore, in a patient presenting with a severe form of primary microcephaly and short stature, we identified compound heterozygous missense mutations in KIF14 (NM_014875.2;c.2545C>G;p.His849Asp and c.3662G>T;p.Gly1221Val). Three of the 5 identified mutations impaired splicing, and 2 resulted in a truncated protein. Intriguingly, Kif14 knockout mice also showed primary microcephaly. Human kinesin-like protein KIF14, a microtubule motor protein, localizes at the midbody to finalize cytokinesis by interacting with CRIK. We found impaired localization of both KIF14 and CRIK at the midbody in patient-derived fibroblasts. Furthermore, we observed a large number of binucleated and apoptotic cells—signs of failed cytokinesis that we also observed in experimentally KIF14-depleted cells. Interpretation: Our data corroborate the role of an impaired cytokinesis in the etiology of primary and syndromic microcephaly, as has been proposed by recent findings on CIT mutations. Ann Neurol 2017;82:562–577.