Background: Professional development (PD), in general, helps teachers enhance their knowledge and skills. Equity-focused PD specifically helps educators understand how personal biases, power structures, and social structures impact their teaching and the larger educational system. Research suggests that incorporating new teaching practices is a gradual process, yet little is known about how effectively these practices are sustained beyond the PD experience. Focus of Study: This study investigated how a critical action research (CAR)--based PD program supported teachers in developing adaptive expertise in culturally responsive education (CRE). By tracking teachers over the two-year PD and one year post-PD, we examined both immediate and sustained changes in their practices. The study addressed the question: How do teachers involved in a critical action research PD develop adaptive expertise in culturally responsive education? Research Design: We used a longitudinal, instrumental case study design to capture the complex and rich nature of teaching and CRE, while also considering the context-dependent circumstances in which it occurs. The longitudinal design allowed us to capture processes and change over time. By collecting interviews and CAR projects from participating teachers throughout the PD and, importantly, for a full year afterward, we were able to investigate both how adaptive expertise develops during a learning experience and the degree to which it continues evolving once the PD experience has ended. Recommendations: Culturally responsive education requires sustained PD rather than brief workshops, emphasizing the need for job-embedded, long-term approaches like CAR. Districts and PD providers should recognize that teachers' adaptive expertise in CRE will vary based on student needs and educational contexts, making prescriptive implementation ineffective. Future research should incorporate longitudinal studies and follow-up support to understand how educators develop and sustain culturally responsive practices over time.