The challenges of addressing increasing calls for the inclusion of computational thinking skills in K-12 education in the midst of crowded school curricula can be mitigated, in part, by promoting STEM learning in after-school settings. The Visualization Basics: Using Gaming to Improve Computational Thinking project provided opportunities for middle school students to participate in after-school clubs focused on game development and LEGO robotics in an effort to increase computational thinking skills. Club leaders and teachers, however, first needed to develop proficiency with the computational tools and their understanding of computational thinking. To achieve these goals, teachers participated in two online professional development courses. After participating in the courses, teachers´ understanding of and attitudes toward computational thinking skills were mostly positive. Observations of club sessions revealed that teachers provided a mix of structured and open-ended instruction. Guided instruction, such as using detailed tutorials for initial exposure to a concept or process, was most commonly observed. One area identified for improvement was the duration of the courses, which provided limited time for teachers to develop deep and robust computational thinking skills. Despite this limitation, the data collected thus far suggest that teachers´ understanding of and attitudes toward computational thinking skills improved.