RCT Randomized controlled trial RELI Receptive-expressive language impairment SES Standardized effect size SLI Specific language impairment SLT Speech and language therapist Studies indicate that language impairment that cannot be accounted for by factors such as belowaverage non-verbal ability, hearing impairment, behaviour or emotional problems, or neurological impairments affects some 6% of school-age children. Language impairment with a receptive language component is more resistant to intervention than specific expressive or phonological delays, and carries a greater risk of comorbid behavioural difficulties as well as adverse outcomes for language development and academic progress. This paper considers underlying explanations that may account for receptive-expressive language impairment. It also reviews evidence for the effectiveness of intervention from theory and recent systematic reviews, trials, and speech and language therapy practice.Children with receptive-expressive language impairment (RELI), also referred to as 'receptive language disorder' 1 or 'mixed receptive-expressive disorder', 2 form a subset of those with speech, language, and communication needs who commonly have problems understanding both spoken and written language. They have particular difficulties in comprehending vocabulary and grammar and inferring meaning and will also have problems with expressive language. Some may have difficulties in pragmatics, i.e. the use of language in social contexts. 3 Population studies indicate that some 6% of 5-year-old children have significant difficulties in language functioning. 4,5 However, variability is observed across studies in the stringency of criteria and the nature of the measures used to define language impairment. 6,7 For example, composite scores are often used which do not distinguish between the child's language production (expressive language skills) and understanding of language (receptive language skills). However, studies that do make this distinction indicate that 2 to 4% of 5-year-olds have RELI. 5,8 A detailed study of referral rates in one primary care trust in the UK (providing local community health services based on 1100 referrals to speech and language therapists [SLTs] over a 15-month period 9 ) would suggest a UK prevalence of 4.5%, which is at the higher end of these estimates. Caution is required, however, as local factors such as staffing and resources for intervention would have influenced referral patterns.RELI is likely to have a marked long-term impact on the outcomes for language development, 10-12 literacy, 10,12,13 behaviour, and social development. 12,14,15 There are links also to mental health problems. 16,17 Studies consistently reveal that RELI is a higher risk factor for adverse long-term outcomes than specific expressive language impairment (ELI), which highlights the importance of effective early intervention.Intervention for RELI may be informed by an understanding of levels of explanation based upon relevant theory and probable underlying mechanis...