The aim of this Campbell systematic review is to examine the effectiveness of speech and language interventions for children with primary speech and language delay/disorder. Thirty‐six studies were found, based on a total of 1,500 children involved in thirty‐three randomised controlled trials. This Review concludes, on the basis of the available international studies in this field, that speech and language therapy generally has a positive effect on children with expressive phonological difficulties. The therapy also helps children who have a problem with their active vocabulary, i.e. children who have difficulty in using words they understand. Children, who have difficulty with active vocabulary alone, but no problem understanding and acquiring language, also benefit from speech and language therapy. Speech and language therapy for children who have difficulty in applying grammar and constructing sentences is, however, only thought to have a positive effect if the child does not have other significant receptive speech and language difficulties. Synopsis Primary speech and language delay/disorder is a common developmental difficulty which, if unresolved, can cause difficulties of both learning and socialisation lasting into adolescence and beyond. This review examines the effectiveness of speech and language therapy interventions for children with primary speech and language delay/disorder. The review concludes that whilst there may be some support for the effectiveness of speech and language therapy for children with expressive phonological and expressive vocabulary difficulties, the evidence concerning the effectiveness of interventions for expressive syntax is mixed, and no evidence is available concerning interventions for children with receptive language difficulties. Abstract Background It is thought that approximately 6% of children have speech and language difficulties of which the majority will not have any other significant developmental difficulties. Whilst most children's difficulties resolve, children whose difficulties persist into primary school may have long‐term problems concerning literacy, socialisation, behaviour and school attainment. Objectives To examine the effectiveness of speech and language interventions for children with primary speech and language delay/disorder. Search strategy The following databases were searched: The Cochrane Controlled Trials Register (Cochrane Library, CENTRAL: 2002/3), CINAHL (1982 – July 2002), EMBASE (1980 – Sept Week 4 2002), ERIC (1965 – 2002), MEDLINE (1966 – Sept Week 3 2002), PsycINFO (1872 – 2002/10 Week 2), The National Research Register (2002/3). In addition to this references were taken from reviews of the literature and reference lists from articles. Selection criteria The review considered randomised controlled trials of speech and language therapy interventions for children or adolescents with primary speech and language delay/disorder. Data collection & analysis Titles and abstracts were identified and assessed for relevance, before the full text version was obtained of all potentially relevant articles. The data were categorised depending on the nature of the control group and considered in terms of the effects of intervention on expressive and receptive phonology, syntax and vocabulary. The outcomes used in the analysis were dependent on the focus of the study with only the primary effects of therapy being considered in this review. Main results The results of twenty‐five studies were used in the meta‐analysis. The results suggest that speech and language therapy is effective for children with phonological (SMD=0.44, 95%CI: 0.01,0.86) or vocabulary difficulties (SMD=0.89, 95%CI: 0.21,1.56), but that there is less evidence that interventions are effective for children with receptive difficulties (SMD=−0.04, 95%CI: −0.64,0.56). Mixed findings were found concerning the effectiveness of expressive syntax interventions (n=233; SMD=1.02, 95%CI: 0.04–2.01). No significant differences were shown between clinician administered intervention and intervention implemented by trained parents, and studies did not show a difference between the effects of group and individual interventions (SMD=0.01, 95%CI: −0.26,1.17). The use of normal language peers in therapy was shown to have a positive effect on therapy outcome (SMD=2.29, 95%CI: 1.11,3.48). Reviewers' conclusions The review shows that overall there is a positive effect of speech and language therapy interventions for children with expressive phonological and expressive vocabulary difficulties. The evidence for expressive syntax difficulties is more mixed, and there is a need for further research to investigate intervention for receptive language difficulties. There is a large degree of heterogeneity in the results, and the sources of this need to be investigated.