This study examined 12 preservice teachers´ understanding of computational thinking while planning and implementing a computational thinking activity for fifth grade students. The preservice teachers were enrolled in an add-on computer education license that would certify them to teach computer courses in addition to their primary major area (11 elementary education majors, 1 secondary social studies education major). The preservice teachers were asked to develop a 2 h instructional project for fifth grade students to build on the computational thinking concepts learned during the 'Hour of Code” activity. Data was collected from preservice teachers´ initial proposals, two blog posts, video recordings of in-class discussions, instructional materials, final papers, and a long-term blog post 3 months after the intervention. Results showcased that the process of developing and implementing computational thinking instruction influenced preservice teachers´ understanding of computational thinking. The preservice teachers were able to provide basic definitions of computational thinking as a problem-solving strategy and emphasized that learning computational thinking does not require a computer. On the other hand, some preservice teachers had misconceptions about computational thinking, such as defining computational thinking as equal to algorithm design and suggesting trial and error as an approach to computational problem solving. We provide recommendations for teacher educators to use more directed activities to counteract potential misconceptions about computational thinking.