This meta-analysis examined communication outcomes in single-case design studies of augmentative and alternative communication (AAC) interventions and their relationship to participant characteristics. Variables addressed included chronological age, pre-intervention communication mode, productive repertoire, and pre-intervention imitation skills. Investigators identified 114 single-case design studies that implemented AAC interventions with school-aged individuals with autism spectrum disorder and/or intellectual disability. Two complementary effect size indices, Tau(AB) and the log response ratio, were applied to synthesize findings. Both indices showed positive effects on average, but also exhibited a high degree of heterogeneity. Moderator analyses detected few differences in effectiveness when comparing across diagnoses, age, the number and type of communication modes, participant's productive repertoires, and imitation skills to intervention. A PRISMA-compliant abstract is available: https://bit.ly/30BzbLv.