A meta-analysis was carried out of interventions for children with primary developmental speech and language delays/disorders. The data were categorized depending on the control group used in the study (no treatment, general stimulation, or routine speech and language therapy) and were 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 aims of the study; only the primary effects of intervention are considered in this review. These were investigated at the level of the target of therapy, measures of overall linguistic development, and broader measures of linguistic functioning taken from parent report or language samples. Thirty-six articles reporting 33 different trials were found. Of these articles, 25 provided sufficient information for use in the meta-analyses; however, only 13 of these, spanning 25 years, were considered to be sufficiently similar to be combined. The results indicated that speech and language therapy might be effective for children with phonological or expressive vocabulary difficulties. There was mixed evidence concerning the effectiveness of intervention for children with expressive syntax difficulties and little evidence available considering the effectiveness of intervention for children with receptive language difficulties. No significant differences were found between interventions administered by trained parents and those administered by clinicians. The review identified longer duration (>8 weeks) of therapy as being a potential factor in good clinical outcomes. A number of gaps in the evidence base are identified.