The purpose of this mixed methods study was to determine the impact of synchronous online tutoring services on struggling middle school students' mathematics achievement. The online tutoring was provided as a response to intervention (RTI) Tier 3 support (intensive, individualized intervention) in schools implementing a school-wide mathematics program that addresses Tier 1 (high-quality classroom instruction) and Tier 2 (small group interventions). We employed quasi-experimental, within- and between-group designs to examine impacts for 119 students in two schools to measure the tutoring's impact on mathematics assessment scores. We also conducted qualitative analyses of student and tutor postsession commentary. The findings suggest that the tutoring contributed to statistically significant gains in student assessment scores postintervention. Online tutors' descriptions of their practice centered on ongoing progress monitoring of student learning, delivery of guided practice to students, the use of multiple explanations and representations of target concepts. Student perceptions of the online tutoring were predominately positive in nature.