STEM outreach programmes in secondary schools are mediated by STEM teachers who are responsible for organising, implementing and evaluating the activities with a view to promoting STEM subjects. However, research investigating teachers' STEM roles and professional development through participation in outreach activities is limited. This paper explores teachers' views of STEM activities, how they understand their role as primary facilitators and the impact of their STEM engagement on their professional development. STEM outreach provides distinct opportunities for situated and dialogic learning. STEM teachers' effectiveness in engaging students by demonstrating the importance and relevance of STEM subjects in the real world and engaging captivating STEM outreach partners not only supports students learning but also enhances their opportunities to develop their knowledge and skills. Semi structured interviews were conducted with eleven teachers from six different schools in the West Midlands, United Kingdom. The data reveals STEM facilitators become the "face" of STEM to their pupils. The expertise, knowledge and partnerships STEM facilitators develop, broadens their capacity to deliver teaching imbued with real world applications and improve students' understanding of the range of new and upcoming careers available. Most importantly, participating in STEM outreach activities help teachers maintain and reinforce their own sense of identity as STEM professionals. Outreach activities provide opportunities for teachers to interact with leading scientists and obtain exposure to cutting-edge research.