The identification of developmental problems in a child's acquisition of speech, language and/or communication is a core activity in child surveillance. These are common difficulties with up to 15% of toddlers being ‘late talkers’ and 7% of children entering school with persisting impairments of their language development. These delays can confer disadvantages in the long term, adversely affecting language, cognition, academic attainment, behaviour and mental health. All children presenting with significant speech and language delay should be investigated with a comprehensive hearing assessment and be considered for speech and language therapy assessment. Socioeconomic adversity correlates with delayed language development. Clinical assessment should confirm that the presentation is definitely not acquired (see part 2) and will also guide whether the difficulty is primary, in which there are often familial patterns, or secondary, from a very wide range of aetiologies. Symptoms may be salient, such as the regression of communication in <3-year-olds which ‘flags up’ autism spectrum disorder. Further investigation will be informed from this clinical assessment, for example, genetic investigation for sex aneuploidies in enduring primary difficulties. Management of the speech and language difficulty itself is the realm of the speech and language therapist, who has an ever-increasing evidence-based choice of interventions. This should take place within a multidisciplinary team, particularly for children with more severe conditions who may benefit from individualised parental and educational supports.