Considering the rapid global movement towards inclusion for students with special educational needs (SEN), there is a surprising lack of pedagogical or didactic theories regarding the ways in which inclusive education may affect students with SEN. Group composition within the educational setting may play a role in determining the academic achievement, socio-emotional development, and wellbeing of students with SEN. Proponents of inclusion propose that segregated educational placement causes stigmatisation and social isolation which may have detrimental effects on the self-concept and self-confidence of students with SEN. On the other hand, opponents of inclusion for all special needs students suggest that placement in general education classrooms may have adverse effects especially if the time and resources allocated for individualisation are not aligned with student needs. Since the 1980s, a number of reviews on the effects of inclusion have been published. Results are inconsistent, and several reviews point to a number of methodological challenges and weaknesses of the study designs within primary studies. In sum, the impact of inclusion on students with SEN may be hypothesised to be both positive and negative, and the current knowledge base is inconsistent. The objective was first: To uncover and synthesise data from contemporary studies to assess the effects of inclusion on measures of academic achievement, socio-emotional development, and wellbeing of children with special needs when compared to children with special needs who receive special education in a segregated setting.A secondary objective was to explore how potential moderators (gender, age, type and severity of special need, part or full time inclusive education, and co-teaching) relate to outcomes. Relevant studies were identified through electronic searches in Academic Search Premier (EBSCO), APA PsycINFO (EBSCO), EconLit (EBSCO), ERIC (EBSCO), International Bibliography of the Social Sciences (ProQuest), Sociological Abstracts (ProQuest), Science Citation Index Expanded (Web Of Science), Social Sciences Citation Index (Web Of Science), and SocINDEX (EBSCO). The database searches were completed on 24 April 2021 and other resources: grey literature repositories, hand search in targeted journals and Internet search engines were searched in August/September 2021. The search was limited to studies reported after 2000. The review included studies of children with special needs in grades K to 12 in the OECD countries. Children with all types of verifiable SEN were eligible. Inclusion refers to an educational setting with a mixture of children with and without SEN. Segregation refers to the separate education of children with SEN. All studies that compared inclusive versus segregated educational settings for children with SEN were eligible. Qualitative studies were not included. The total number of potentially relevant studies constituted 20,183 hits. A total of 94 studies met the inclusion criteria, all were non-randomised studies. The 94 studies analysed data from 19 different countries. Only 15 studies could be used in the data synthesis. Seventy-nine studies could not be used in the data synthesis as they were judged to be of critical risk of bias and, in accordance with the protocol, were excluded from the meta-analysis on the basis that they would be more likely to mislead than inform. The 15 studies came from nine different countries. Separate meta-analyses were conducted on conceptually distinct outcomes. All analyses were inverse variance weighted using random effects statistical models. Sensitivity analyses were performed to evaluate the robustness of pooled effect sizes across components of risk of bias. The average baseline year of the interventions analysed in the 15 studies used for meta-analysis was 2006, ranging from 1998 to 2012. The average number of participants analysed in the interventions was 151, ranging from 10 to 1357, and the average number of controls was 261, ranging from 5 to 2752. The studies included children with multiple types of disabilities such as learning disorders/intellectual disabilities, autism spectrum disorders, ADHD, physical handicaps, visual impairments, and Down syndrome. At most, the results from eight studies could be pooled in any of the meta-analyses. All the meta-analyses showed a weighted average that favoured the intervention group. None of them were statistically significant. The random effects weighted standardised mean difference was 0.20 (95% confidence interval [CI]: -0.01 to 0.42) for overall psychosocial adjustment; 0.04 (95% CI: -0.27 to 0.35) for language and literacy learning outcomes, and 0.05 (95% CI: -0.16 to 0.26) for math learning outcomes. There were no appreciable changes in the results as indicated by the sensitivity analyses. There was some inconsistency in the direction and magnitude of the effect sizes between the primary studies in all analyses and a moderate amount of heterogeneity. We attempted to investigate the heterogeneity by single factor sub-group analyses, but results were inconclusive. The overall methodological quality of the included studies was low, and no experimental studies in which children were randomly assigned to intervention and control conditions were found. The 15 studies, which could be used in the data synthesis, were all, except for one, judged to be in serious risk of bias. Results of the meta-analyses do not suggest on average any sizeable positive or negative effects of inclusion on children's academic achievement as measured by language, literacy, and math outcomes or on the overall psychosocial adjustment of children. The average point estimates favoured inclusion, though small and not statistically significant, heterogeneity was present in all analyses, and there was inconsistency in direction and magnitude of the effect sizes. This finding is similar to the results of previous meta-analyses, which include studies published before 2000, and thus although the number of studies in the current meta-analyses is limited, it can be concluded that it is very unlikely that inclusion in general increases or decreases learning and psychosocial adjustment in children with special needs. Future research should explore the effects of different kinds of inclusive education for children with different kinds of special needs, to expand the knowledge base on what works for whom.