BACKGROUND:Adolescents in contact with youth justice are a vulnerable and marginalized group at high risk of developmental language disorder (DLD) and other communication difficulties. Though preliminary studies have demonstrated the benefits of speech and language therapy (SLT) services in youth justice, limited research has empirically tested the efficacy of intervention in these settings. AIMS:To evaluate the extent to which intensive, one-to-one language intervention improved the communication skills of incarcerated adolescents with below-average (> 1 SD below the mean) language and/or literacy skills. METHODS & PROCEDURES:A series of four empirical single case studies was conducted, using multiple baseline intervention design. Individualized intervention programmes were administered, and progress on outcome measures (probes) was evaluated throughout the baseline, intervention and maintenance phases using Tau-U, a non-parametric distribution-free statistic. Additional measures were used as secondary outcomes of the intervention, including standardized language subtests, subjective rating tools by participants and their teachers collected pre- and post-intervention, and a brief structured participant interview, independently administered by youth justice staff. OUTCOMES & RESULTS:Medium-to-large effect sizes, the majority of which were statistically significant, were detected on the primary outcome measure across the four cases, indicating improvements in the targeted communication skills. Positive results were also evident in comparisons of pre- and post-measures on standardized language subtests, subjective self- and teacher ratings of communication, and the participants' impressions of the interventions. For those participants who could be followed up, gains in language skills were generally maintained at 1 month post-intervention. CONCLUSIONS & IMPLICATIONS:This study provides further evidence of the efficacy of one-to-one SLT intervention for adolescents in youth justice in order to address language and literacy difficulties. These findings inform future SLT service provision for adolescents in these settings, with clear policy and practice implications. Future research should investigate the wider benefits to individuals' engagement in youth justice intervention and recidivism, as well as assessing maintenance of gains over a longer period. What this paper adds What is already known on this subject The high rates of DLD in youth justice is well known, with difficulties spanning multiple areas of language and literacy. SLTs are increasingly working in community and custodial youth justice settings, and a few preliminary studies have demonstrated the effectiveness of such work. What this paper adds to existing knowledge This study extends the evidence base of the efficacy of SLT for language and literacy difficulties in youth justice, using a series of four empirical single case studies. It is also argued that SLT should be more actively considered in planning multidisciplinary interventions for young people in custody. What are the potential or actual clinical implications of this work? The results of this research support current moves to include SLT services in youth justice systems, and illustrate for clinicians currently working in this sector a way of structuring and measuring the impact of intervention services.