International comparisons in science (and mathematics) education show the relevance of teaching quality for learning outcomes. Teacher professional development (TPD) is hence considered particularly relevant for improving teaching and learning in science. The study at hand is part of a European network initiative named Mind the Gap that explores the conditions of TPD. In our project, expert interviews were used to build a knowledge base that includes both the status quo and challenges of TPD in European countries. As a method, we used semi-structured interviews with designated experts from five European countries. The interview questions were based on criteria identified by research on effective TPD conditions. The results of qualitative analyses reveal specific patterns of TPD in groups of European countries, with some countries matching effective TPD conditions closer than others. Experts from all five countries (Denmark, Germany, Hungary, Norway and the United Kingdom) identified basic organizational and monetary support for participating in TPD as a major challenge for future development. (Contains 1 figure and 3 tables.)