Teaching mathematics can be challenging and requires depth and breadth of both content and pedagogical knowledge unique to the subject. Teaching students experiencing mathematical difficulties (MD) requires additional knowledge and skills related to implementing data-driven instructional practices. Professional development has been shown to be an effective method of improving teacher knowledge and skills related to both mathematics and data-driven practices. In a systematic review of the literature on mathematics PD, however, I found variability in the most effective structures and methods for delivering PD. This study examined the effects of a 9-week intensive summer PD regarding the use of data-based individualization within mathematics classrooms on teachers’ mathematical knowledge for teaching, self-efficacy, and instructional practices. A total of 48 initial participants were randomly assigned to complete the summer PD either as part of an Asynchronous group, a Synchronous/Independent group, or a Synchronous/PLC group. Participants completed the National Center on Intensive Intervention (NCII) Mathematics Intervention Course over the course of the 9 weeks, completing approximately 40 hours of materials and activities. ANOVA tests yielded significantly positive results showing participants improved on the Mathematics Teaching Self-Efficacy measure as well as the Teachers Instructional Practices (TIP) measure. Results also showed positive but not significant improvements on Mathematical Knowledge for Teaching (MKT) as well as s Teacher’s Sense of Efficacy Scale. Although not significantly different, participants in the Synchronous groups (i.e., Synchronous/Independent and Synchronous/PLC) saw greater improvements than participants in the Asynchronous group on all measures, and participants in the Synchronous/PLC group saw greater improvements than participants in the Non-PLC groups (i.e., Asynchronous group and Synchronous/Independent group) on all measures. Results of thematic analysis of semi-structured interviews also suggested that participants valued the PD for resources, accountability, and a boost to their confidence, although experiences qualitatively differed across groups. These findings provide initial support for the use of the NCII Mathematics Intervention Course as the foundation for future PD and the inclusion of peer support and accountability structures for online PD. Future research might investigate further the most effective ways of supporting participants in completing the NCII Mathematics Intervention Course during the school year or with observations and coaching support to ensure long-lasting instructional change.