Teachers play a key role in preparing future generations of transversally skilled people and are thus pivotal in the context of green and digital transitions. Despite this, teachers report a lack of professional development opportunities, esp. related to digital skills, mobility, and peer mentoring. The goal of acaSTEMy is to develop a systemic support structure for high-quality, research-based STEM teacher education from pre-service education to continuing professional development (CPD) that includes mobility as an essential building block. The project envisions transversally competent and motivated STEM teachers who are well equipped to prepare their students for future careers, the nature of which is difficult to foresee. Building on numerous previous activities of acaSTEMy partners, the focus of the project is to: (1) develop, pilot and promote various distance and blended learning models in combination with international mobility and mentorship; (2) improve STEM teachers’ digital competences for meaningful pedagogies; (3) support the competences and pedagogies of STEM teachers for sustainable and up-to-date education by developing CPD courses to address major challenges, such as environmental sustainability, green deal, global health and immigration as well as methodological aspects, such as teaching diverse classes, combating science anxiety, and fostering gender-sensitive teaching. AcaSTEMy’s bottom-up alliance brings together providers of pre- and in-service teacher education, as well as practice schools from eight (8) countries to develop and test mobility models and programmes for effective and accessible professional learning. In addition, the partnership network includes STEM teacher associations, ministries of education and Academies of Sciences to provide input for developing a policy framework for purposeful and systematic teacher mobility and to inform broader science education policies at national and European levels.