Building upon previous research that focused on the relationships between job demands, job resources, and employee psychological well-being, this longitudinal research makes a unique contribution by relating job demands and job resources to teachers’ professional development (TPD) at work and flexible competence, the latter being a key factor in teachers’ career development. This study was carried out among 211 teachers working in primary and secondary education in the Netherlands. TPD at work appeared to be related to flexible competence and proved to be a mediator between job resources, on the one hand, and flexible competence, on the other hand. Job resources positively enhanced TPD at work and in turn were related to flexible competence. Moreover, a direct negative relationship between job demands and flexible competence was found.