Background: Increasing numbers of children with behaviour and school problems (related to both academic achievement and social participation) are recognized as having undiagnosed speech, language and communication difficulties. Both speech, language and communication difficulties and school failure are risk factors for offending. Aims: To investigate the prevalence of speech, language and communication difficulties in a group of persistent and prolific young offenders sentenced to the Intensive Supervision and Surveillance Programme (ISSP), and to provide a preliminary evaluation of the impact of speech and language therapy (SLT) intervention. Methods & Procedures: Seventy-two entrants to ISSP over 12 months were screened by the speech and language therapist. Those showing difficulties then had a detailed language assessment followed by intervention delivered jointly by the speech and language therapist and the youth offending team staff. Reassessment occurred at programme completion. Outcomes & Results: A total of 65% of those screened had profiles indicating that they had language difficulties and might benefit from speech and language therapy intervention. As a cohort, their language skills were lower than those of the general population, and 20% scored at the ‘severely delayed’ level on standardized assessment. This is the first study of speech and language therapy within community services for young offenders, and is the first to demonstrate language improvement detectable on standardized language tests. However, further research is needed to determine the precise role of speech and language therapy within the intervention programme. Conclusions & Implications: Children and young people with behavioural or school difficulties coming into contact with criminal justice, mental health, psychiatric, and social care services need to be systematically assessed for undiagnosed speech, language and communication difficulties. Appropriate interventions can then enable the young person to engage with verbally mediated interventions.