Due to the introduction of mass customization, there is a high need for multi-skilled operators that can perform a wide range of tasks. However, finding the right trade-off between cost and benefit is crucial to remain competitive. Next to the development of operators, a manufacturing company needs to cope with absenteeism, changes in product mix and volumes, and unforeseen circumstances. This PhD research aims to investigate (1) how to assign workers to tasks and sequence jobs to maximize operator learning and minimize operator forgetting, (2) how to model operator competency and its evolution most efficiently and realistically, (3) how to consider the effect of off-the-job training assignments, (4) how to consider the trade-off between operator development and assembly efficiency and (5) how to solve the resulting model.