Online professional development (OPD) for teachers presents a wide range of opportunities and challenges for professional development (PD) designers and teachers alike. While OPD offers affordances such as the ability to overcome geographical barriers, it is often delivered in a “one size fits all” manner that fails to meet teachers’ diverse needs. Interest development (Hidi & Renninger, 2006) provides one promising lens for understanding, supporting, and designing for individual teacher needs within OPD. However, there is a need for more research to understand the precise individualized mechanisms of long-term interest development and its manifestations within OPD contexts. In this study, I applied multiple case study methodology to understand the experiences of three high school science teachers as they engaged with an OPD program over the course of three years. I applied an iterative process of qualitative data analysis to draw meaning from a variety of data sources, including discussion forum participation, interviews, and communications with the research team. My analysis was guided by the following research questions: (1) how did teachers’ interest development evolve over the extended period of time that they engaged with our OPD programming; and (2) what individualized mechanisms and supports contributed to and impacted the teachers’ interest development during this time. In drawing comparisons across the three cases, nine themes emerged regarding the teachers’ interest development within the BioGraph OPD: (1) clear conveyance of PD utility helped trigger and support early levels of situational interest; (2) adaptability of PD programming and alignment with teachers’ personal goals supported maturation of well-developed individual interest; (3) timely communication supported relationship building and self-efficacy development; (4) opportunities to give back helped to fortify individualized interest; (5) perception that efforts were appreciated helped maintain and promote interest development; (6) reciprocated effort by the PD facilitation team supported long-term individual interest; (7) misalignments in peer interest-related needs were alleviated through active facilitation; (8) personal relationships in the online learning space were impactful for long-term engagement; and (9) engagement with the PD and community continued to evolve over a multi-year timeframe.