<p>While transitions occur multiple times over the course of medical training, there is limited research focused on fellows’ experiences and challenges during their transition from residency into subspecialty training, specifically hematology-oncology fellowship. We aimed to explore the lived experiences of hematology-oncology fellows, with a focus on the transition to fellowship. We conducted a qualitative study using one-on-one semi-structured interviews with 13 hematology-oncology fellows across all three fellowship years at the University of Colorado from September 2024 to May 2025. The authors used a combined interpretive phenomenological analysis approach and practical thematic analysis to describe hematology-oncology fellows’ experiences of the fellowship transition. Fellows describe three distinct transitional phases including 1) pre-fellowship, 2) initial transition, and 3) extended transition. The end of the transition is followed by a plateau/stabilization phase. These phases are characterized by different levels of autonomy, support needs, and learning, and are influenced by fellows’ self-imposed expectations and program and attending expectations. This qualitative study exploring fellows’ experiences during their first year of hematology-oncology fellowship provides insight into the challenges of adapting to new roles in subspecialty training.</p>