Background: A significant proportion of adults with intellectual disabilities (ID) experience speech, language and communication difficulties which are associated with poor physical and mental health outcomes. Speech and language therapy (SLT) interventions are an important way to address these communication difficulties, yet there is limited available evidence to provide information about the effectiveness of the different approaches used for this heterogeneous group. Aims: To review the evidence available for the effectiveness of SLT interventions aimed at improving communication for adults with ID. Methods & Procedures: A systematic search across relevant databases was performed. Information on methodological details of each relevant study, along with descriptions of the SLT interventions employed, were extracted and the Crowe Critical Appraisal Tool (CCAT) was used to assess quality. Findings were discussed in a narrative synthesis grouped by target communication skill. Outcomes & Results: A total of 10 relevant studies met the inclusion criteria. These were predominantly interventions aimed directly at adults with ID to improve speech, increase augmentative and alternative communication (AAC) use and develop interaction skills, with one study addressing work with carers. The included studies were all rated as low quality. There is weak preliminary evidence that SLT input can improve the communication skills of adults with ID. Conclusions & Implications: There is insufficient evidence to draw strong conclusions about the effectiveness of SLT in this population. Further high-level evidence across speech, language and communication domains is urgently needed.