Background: In times of accelerating changes, teachers who proactively engage in activities towards school improvement and innovation are increasingly needed. Still, studies on factors that affect teachers' proactive behaviour are rare. Aims: Integrating previous research on proactive behaviour within the Job Demand--Resources (JD-R) Model, this paper investigates how job characteristics ("time pressure," "bureaucratic structures," "participative climate," "personal initiative of the team") and aspects of teachers' professional competence ("self-efficacy," "self-regulation skills" and "knowledge") contribute to and interact with their proactive behaviour. Sample: A total of 130 German secondary school teachers (M(SD)[subscript age] = 44.05 (11.36), 65% female) participated in this study. Methods: We employed a full two-wave panel design, with measurement points 5 months apart. The data were analysed with (moderated) single indicator modelling and a cross-lagged panel model. Results: While teachers' self-efficacy in implementing change and self-regulation skills predicted their concurrent proactive behaviour, job characteristics and teachers' knowledge had no such cross-sectional effects. In addition, we found an interaction effect of time pressure and teachers' self-efficacy on proactive behaviour. Including the second measurement point, data indicated no cross-lagged effects of the job and personal factors on proactive behaviour. However, cross-lagged analysis revealed that teachers' proactive behaviour predicted their later self-efficacy in implementing change and the time pressure they perceive. Conclusions: Examining both cross-sectional and longitudinal effects, this study highlights the importance of measurements over time when analysing factors that influence teachers' proactive behaviour: While aspects of professional competence appear to be trainable concurrent resources, time pressure can limit their effect. Finally, cross-lagged effects of teachers' proactive behaviour on their later self-efficacy and time pressure appear as influential in the long run.