A Cochrane review (abstract , review ) included 33 studies. The results of a meta-analysis of 25 studies suggest that speech and language therapy is effective for children with phonological (SMD=0.44, 95% CI 0.01 to 0.86) or vocabulary difficulties (SMD=0.89, 95% CI 0.21 to 1.56), but that there is less evidence that interventions are effective for children with receptive difficulties (SMD=-0.04, 95% CI -0.64 to 0.56).
Comment: The quality of evidence is downgraded by study quality (inadequate or unclear allocation concealment and small sizes) and by inconsistency (variability in results across studies).