Continuous Professional Development (CPD) can be crucial for reforming science teaching, but more knowledge is needed about how to support sustainability of the effects. The Danish QUEST project is a large scale, long-term collaborative CPD project designed according to widely agreed criteria for effective CPD. The QUEST-project is divided into two main phases: implementation and institutionalization. All activities in the implementation phase were organized in a rhythm of seminars followed by local individual and collaborative inquiries. The paper presents findings from this two-year implementation phase. The findings are discussed looking forward to the institutionalization phase identifying factors potentially supporting sustainable development pertaining to local science teachers developing a shared focus on student learning in science, and perceived individual and collective efficacy. The participants rated their overall benefits from phase 1 as rather high. They reported that they tried out models, tools and activities from the course modules in their own classrooms, and in collaboration with colleagues. This is confirmed by observations. In relation to developing a collaborative culture in the local learning communities we have seen a positive trend from the first to the last course-module, but with great variation between schools. Factors potentially supporting sustainable development seem to be about (1) continuingly scaffolding teachers’ collaborative inquiries by organizing activities following the QUEST-rhythm, (2) securing the meeting between research-based knowledge and practitioner knowledge by transforming innovative ideas from research into concrete tools in collaboration with participating teachers, and (3) supporting teachers’ enactive mastery experiences.