Mutations in CDK13 have recently been identified as a novel cause of syndromic intellectual disability. In this chapter, we review the 44 cases of CDK13-related disorder reported to date, highlighting key clinical pointers to this diagnosis including characteristic craniofacial features, feeding difficulties in infancy, and the presence of structural heart or brain malformations. The spectrum of reported mutations is also described, demonstrating an excess of missense mutations arising in the protein kinase domain. Exploration of genotype-phenotype correlations suggests a trend toward milder phenotypes in patients with mutations predicted to cause haploinsufficiency of CDK13, while missense mutations affecting amino acid residue 842 appear most likely to be associated with structural malformations. The greater phenotypic impact of missense variants is hypothesized to occur due to a dominant-negative mechanism, by which the mutant protein acts to sequester cyclin K in inactive complexes. Functional studies to validate this hypothesis have not yet been carried out, however. Differential diagnosis and recommendations for clinical care of patients with CDK13-related disorder are also described, emphasizing baseline echocardiography, vigilance for feeding and swallowing difficulties, and regular developmental evaluation as key components of care. Finally, future directions for CDK13 research are discussed, including the need to resolve uncertainty regarding pathogenicity of CDK13 haploinsufficiency, and to gather further longitudinal data from large cohorts in order to inform the clinical care of patients with this diagnosis.