In-service teacher development programs are perceived as a common way to foster school improvement. Nevertheless, program evaluation to date ends with the assessment of acquired knowledge, skills, and motivation, frequently disregarding the actual learning transfer and its conditions. This study aimed to investigate how individual and situational factors interact and contribute to learning transfer in an in-service teacher development program. We tested a transfer model derived from human resources development research. One hundred and twenty-nine teachers aged 25-62 years (87% female), participating in an in-service development program, were surveyed three times during the program. Results of path analyses suggest that not only especially individual factors are critical for learning transfer in the context of school improvement, but also that program and environmental factors have important effects. They moderated the effects of the individual factors.