Background <p>Despite a 75-year history of building epidemiologic capacity and strengthening public health systems, the learning processes in field epidemiology training programmes (FETPs) remain unexamined.</p> Methods <p>We codesigned a grounded theory and narrative inquiry qualitative study to fill this gap. The study aimed to understand the learning processes in four FETPs by describing training approaches for field epidemiologists, outlining learning strategies among trainees, and examining principles and practices that align training approaches and learning strategies. Data collection included participant observations and semi-structured interviews with FETP trainees and advisors within programmes in Australia, Japan, Mongolia, and Taiwan.</p> Results <p>Analysis revealed that learning occurs as trainees engage in real-world public health contexts, interacting with their people, systems, data, and knowledge. Facilitators of the learning process were learning environments (projects, routine placement work, field investigations, and courses), advisor stewardship, and trainee tenacity.</p> Conclusions <p>Our findings align with established and contemporary learning theories and suggest that all countries have the tools to build field epidemiology capacity and leadership. To refine these tools, governments, partners, and programme leaders should ensure access to learning environments, fortify advisor stewardship, and foster a culture of resilience among trainees. FETP is among the strongest levers to bolster the workforce for global health security before the next pandemic, and these findings reveal pathways toward better investments.</p>