Although quality guidelines for single-case intervention research emphasize the importance of concurrent baselines in multiple-baseline and multiple-probe designs, nonconcurrent variations on these designs persist in the research literature. This study describes a systematic review of special education intervention studies (k = 406) between 1988 and 2020 that report using nonconcurrent multiple-baseline or multiple-probe designs to test interventions for individuals with disabilities ages 21 years and younger. We coded and synthesized study characteristics pertaining to participants, settings, interventions, data reporting, and contextual factors. Findings indicate the prevalence of nonconcurrent designs for intervention studies in a variety of settings addressing social and communication needs of individuals with disabilities. We discuss implications for research and practice and offer suggestions for improving the validity of nonconcurrent designs. A PRISMA-compliant abstract is available at https://osf.io/sdnj5/?view_only=f386b1fe5f14430a8d63fceed293718d.