Children with speech sound disorder show diminished accuracy and intelligibility in spoken communication and may thus be perceived as less capable or intelligent than peers, with negative consequences for both socioemo- tional and socioeconomic outcomes. While most speech errors resolve by the late school-age years, between 2-5% of speakers exhibit residual speech errors (RSE) that persist through adolescence or even adult- hood, reflecting about 6 million cases in the US. Both affected children/families and speech-language pathologists (SLPs) have highlighted the critical need for research to identify more effective forms of treatment for children with RSE. In a series of single-case experimental studies, our research team has found that treatment incorporating technologically enhanced sensory feedback (visual-acoustic biofeedback, ultrasound biofeedback) can improve speech in individuals with RSE who have not responded to previous intervention. A randomized controlled trial (RCT) comparing traditional vs biofeedback-enhanced intervention is the essential next step to inform evidence-based decision-making for this prevalent population. Larger-scale research is also needed to understand heterogeneity across individuals in the magnitude of response to biofeedback treatment. The overall objective of this proposal is to conduct clinical research that will guide the evidence-based man- agement of RSE while also providing novel insights into the sensorimotor underpinnings of speech. Our central hypothesis is that biofeedback will yield greater gains in speech accuracy than traditional treatment, and that individual deficit profiles will predict relative response to visual-acoustic vs ultrasound biofeedback. Building on a robust history of recruiting and treating children with RSE, we will enroll n=118 children who misarticulate the /r/ sound, the most common type of RSE. Aim 1 will evaluate the efficacy of biofeedback relative to traditional treatment in a well-powered randomized controlled trial (Correcting Residual Errors with Spectral, ULtrasound, Traditional Speech therapy, or C-RESULTS). Ultrasound and visual-acoustic biofeedback, which have similar evi- dence bases, will be represented equally; comparison between biofeedback types is deferred to Aim 3. Second, we will measure auditory and somatosensory acuity in participants with RSE and a matched sample of 100 typical children, allowing us to identify sensory deficit profiles that could act as biomarkers to predict treatment response in RSE. Finally, Aim 3 will select a subset of 8 children with RSE who show a deficit in one domain (auditory or somatosensory) and intact perception in the other. Single-case methods will be used to test the hypothesis that sensory deficit profiles differentially predict response to visual-acoustic vs ultrasound biofeedback. This research will be clinically significant because Aim 1 will have immediate implications for the selection of the most efficient/effective therapy ...