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Global developmental delay (GDD) affects 1%–3% of the population of children under 5 years of age, making it one of the most common conditions presenting in paediatric clinics; causes are exogenous, genetic (non-metabolic) or genetic (metabolic). Recent advances in biotechnology and genetic testing mean that the investigations available to perform for children under 5 years are increasing and are more sensitive than previously. This change in availability and type of testing necessitates an update in the recommendations for investigating GDD.We conducted a review of the literature from 2006 to 2016 to identify articles with evidence relating to the investigation of developmental delay in children under the age of 5 years. We collated the evidence into first-line and second-line investigations and, where available, on their yield and cost implications.We have provided up-to-date guidance for first-line and second-line investigations for children with GDD under the age of 5 years. Recent evidence demonstrates that genetic testing for all children with unexplained GDD should be first line, if an exogenous cause is not already established. Our review of the literature demonstrates that all patients, irrespective of severity of GDD, should have investigations for treatable conditions. Evidence demonstrates that the yield for treatable conditions is higher than previously thought and that investigations for these metabolic conditions should be considered as first line. Additional second-line investigations can be led by history, examination and developmental trajectories.We may need to update present recommendations in the UK for investigation of developmental delay. This would include microarray testing as first line and a more thorough approach to investigations for metabolic disorders that can be treated. Clinical assessment remains vital for guiding investigations.