Language disorder is characterized by difficulty with the comprehension and production of different aspects of language. School-aged children with developmental language disorder (DLD) and school-aged children with Down syndrome (DS) demonstrate similar deficits in the area of morphosyntax, which often creates barriers during social interactions and for academic learning. Although affected children may receive intervention, progress can be slow. Selecting appropriate treatment targets is critical for effective intervention. Traditional approaches for selecting treatment targets are based on developmental readiness (i.e., developmental) or functional need (i.e., remedial). However, intervention utilizing targets selected by these methods produce only modest results, despite a large investment of time. The Complexity Account of Treatment Efficacy (CATE) is a novel approach emerging in morphosyntactic intervention that suggests treating a more complex target may result in progress for simpler, yet related, structures without direct intervention on those related forms. Yet, few investigations have examined the efficacy of using CATE to guide target selection for morphosyntactic treatment. The primary purpose of this study was to evaluate the efficacy of using a complexity-based target to treat morphosyntactic deficits in children with DLD or with DS. The extent to which treatment resulted in generalization to untreated simpler morphosyntactic structures and possible transfer to naturalistic communication contexts was also examined. Perspectives of participant and family experiences during treatment were evaluated to determine the extent to which the treatment was acceptable. Three participants with DLD and three with DS received treatment in a single-subject experimental design study. Accuracy with the treated form was measured repeatedly during the baseline phase, which continued throughout treatment and post-treatment phases to identify improvements associated with treatment. Outcomes indicated treatment of a complex form resulted in improved accuracy with the targeted form and generalization to untreated simpler morphosyntactic structures for some participants. Transfer to naturalistic contexts was also observed. Feedback regarding the acceptability of treatment for families was generally positive. The results of the study provided evidence supporting the efficacy of using a complexity-based approach for selecting targets for morphosyntactic treatment. Additional research is needed to identify specific characteristics that might predict individual responses to treatment. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]