This article describes the effectiveness and sustainability of teacher professional development interventions from a complexity viewpoint as well as a more "standard" viewpoint. The first aim of this study is to give a theoretical overview of effective aspects of interventions regarding teachers' professionalizing using recent literature. The second aim is to re-interpret effectiveness and effectiveness studies using a complexity approach. The third aim is to empirically illustrate a complexity approach to the effectiveness of interventions using a multiple case study. We have described intervention specific aspects, teacher specific aspects, context specific aspects and implementation specific aspects and have shown in the cases that during an intervention these aspects intertwine and act as a complex dynamic system. The complexity approach to interventions has implications for future empirical research as well as for the design of interventions. In future research, results of small scale and large scale research should be combined, in order to obtain a better insight in all relevant aspects and their effect on teachers' behavior. Research ought to concentrate on the effects that all contextual aspects have intertwiningly in order to design more effective interventions and with better implementation processes.