Background: Online CPD programmes may be able to assist the Saudi Arabian Ministry of Education in enhancing teaching standards and learning outcomes while overcoming the barriers to implementation, which face-to-face (f2f) CPD programmes can encounter. Yet, little research has compared the effectiveness or differences between the online and f2f delivery of CPD within the Saudi context. Purpose: This study aims to undertake a "comparative evaluation study" of the impact of f2f and online CPD programmes on science teachers' pedagogical practice. Sample: Twenty male science teachers, from different schools in Saudi Arabia, were selected to participate in this study. The sample was divided into two groups (face-to-face and online), each made up of 10 teachers. Design and methods: The study used a mixed methods approach incorporating: (1) Classroom observations using the Flanders Interaction Analysis Category (FIAC) system of classroom observational analysis; (2) A questionnaire survey of all teacher participants; and (3) semi-structured interviews. Results: The findings of the study suggest that the online CPD was as effective as, and in certain places more effective than, the f2f CPD in terms of developing conceptual understanding. Furthermore, the overall satisfaction was found to be more positive amongst those teachers undertaking the online CPD compared to those undertaking the f2f CPD. Conclusion: These findings have important implications for educational policymakers as it shows that the use of online programmes is not only as effective, and in some cases more so, in terms of CPD training provision than traditional f2f but is easier to deliver. As such it provides an opportunity to expand the provision of CPD both across large geographical areas where f2f CPD might be traditionally difficult to access as well as in those countries in which social norms prevent direct f2f interaction between members of the opposite sex.