The 21st Century Skills and Competences for New Millennium Learners have been shaping curriculum reforms and educational trends around the world, including in Portugal. In this article we discuss a theoretical perspective on integrated STEM education in which practices have a central place. We consider the current importance of developing professional development programs for teachers, including mathematics, based on a conceptual framework for promoting scientific, technical, and technological knowledge and practices. Our aim is to know how mathematics teachers experience STEM activities and how they evaluate the results of their implementation in the classroom. The study uses a qualitative methodology, in the context of a professional development program. The results show that teachers performed STEM practices in a proposed task and positively evaluated the effects of their students' work on the task, namely in promoting 21st century skills.