To deal with the increasingly complex reality in class, which has accelerated in the last years, teachers need support through professional development (PD) activities to adopt new integrative methods. This need is particularly pronounced in STEM teaching (Science, Technology, Engineering and Mathematics) since STEM is a field in which all pupils, including migrants or those from disadvantaged backgrounds, should acquire appropriate skills in order to ensure their participation in work and life. Nowadays, the need for high-quality STEM teacher PD and the promotion of approaches that allow teachers to deal with complex classroom realities has been recognized. This can be seen in policy documents and reports such as the new Science Education for Responsible Citizenship (2015) report. It is also evidenced by the recent trend in Europe to establish PD centres to promote STEM teacher PD. What has not yet been achieved is to deliver the highest possible quality, based on findings from research, practice, different contexts, subjects and countries. Too often, therefore, PD activities do not lead to necessary changes in classroom practice. Hence, our overall aim is to improve the quality and relevance of STEM teacher PD on a large scale, and to do so by strengthening the position, work and knowledge base of STEM PD centres across Europe: through transnational exchange and mutual learning in a stable European PD centre network, through linking research with practice, through developing urgently required ready-to-use guidelines and reference materials for STEM PD providers. To have the widest possible impact specialized STEM PD centres will be working together in this project and set up a European network. They are united by their mission to improve STEM teacher PD and bring together a wide range of different expertise. On the basis of a prior informal network, two European meetings and a survey among PD centres, we have identified the needs and requirements to effectively increase the quality of STEM PD delivery across Europe. Based on this we will produce and disseminate:• Quality criteria for PD and guidelines to day-to-day implementation • Ready-to-use guidelines on measuring the quality of a PD activity • A catalogue for PD providers to take advantage of today’s diverse classrooms• A commented selection of STEM PD materials of the highest quality available across Europe• A review of PD centre models, their challenges and success factors leading to improvements in STEM teacher PD delivery of those centres• An interactive web-portal • Policy briefings on core concerns, insights and outcomes of the network Our work comprises five main strands of activities: (1) Development of the intellectual outputs; (2) dissemination activities; (3) working on the sustainability of the PD centre network; (4) quality assurance and evaluation and (5) project management. Quality assurance and evaluation as well as project management are built into the other working strands and thus ensure maximized output and impact. Our intellectual outputs are designed relying on the expertise of 12 different STEM PD centres and 2 schools that jointly will ensure quality and relevance of our results. All outputs are based on a substantial preparation phase (e.g. literature research), a development and design phase (e.g. linking research with practice needs and contexts) and a drafting, reviewing and optimizing phase (cf. quality assurance). Transnational learning activities serve the two-fold purpose: to further enhance our outputs while providing learning and exchange for the benefit of organizational improvements among PD centres. Our activities to enhance impact will start right from the beginning of the project (through e.g. web publications, tweets, newsletters, workshops, enlarging contacts database). They support dissemination throughout the project; in particular the broad reception of our major intellectual outputs will be supported through (inter)national multiplier events.The joint development of PD guidelines, compendia, materials, mutual learning activities, dissemination activities and multiplier events will support the organizational development of STEM PD centres in the network, improve the quality of their offers and strengthen their role as leveraging institutions in national contexts. Our activities will as well impact on PD centres beyond the network – and thereby invigorating the role of PD centres in general and strengthening the voice of PD practice in Europe vis-à-vis policy, research and the public. Through our approach the project will significantly improve the quality of STEM PD delivered across Europe. Teachers will be better prepared to deal with complex and highly diverse realities they encounter in classrooms today. This will in turn be to the benefit of European pupils. Our European network that will be enlarged and continued during/after the project’s duration provides for long-term benefit.