This chapter investigates the five-decade evolution of teacher in-service training in Indonesia. Based on literature review of key academic and policy-related sources, this chapter attempts to trace the historical development of the professional development initiatives. The analysis highlights two distinct periods, namely, the centralized New Order (1970s–1990s) and the decentralized Reform Era (2000s–present). Teacher in-service training during the New Order era was characterized by large-scale state initiatives such as PKG and SPKG assisted with international support. In contrast, the decentralized reform era was marked by numerous efforts such as CLCC project, MGMP groups, and the Guru Penggerak program. Although the programs in the New Order era had broad reach, they struggled with coordination and limited local engagement. Whereas in the Reform Era, decentralization was brought, shifting authority to local governments and resulting in varied, often fragmented training efforts. The introduction of teacher certification program in 2007 not only marked a significant policy shift but also revealed challenges and gaps in program quality and inclusivity. It has been very common across all eras with cascade training models and a consistent emphasis on student-centered learning. This chapter reveals that the impact of the training programs remains hindered due to persistent challenges such as Indonesia’s vast geography, bureaucratic inefficiencies, and inconsistent implementation of the in-service teacher training programs. Finally, this chapter argues that while in-service training remains vital, deeper systematic changes in terms of how teachers are prepared, supported, and evaluated are needed to truly uplift the quality of teachers.