This study examined longitudinal relationships between job resources, personal resources, and work engagement. On the basis of Conservation of Resources theory, we hypothesized that job resources, personal resources, and work engagement are reciprocal over time. The study was conducted among 163 employees, who were followed-up over a period of 18 months on average. Results of structural equation modeling analyses supported our hypotheses. Specifically, we found that T1 job and personal resources related positively to T2 work engagement. Additionally, T1 work engagement related positively to T2 job and personal resources. The model that fit best was the reciprocal model, which showed that not only resources and work engagement but also job and personal resources were mutually related. These findings support the assumption of Conservation of Resources theory that various types of resources and well-being evolve into a cycle that determines employees' successful adaptation to their work environments. (Contains 3 tables and 1 figure.)