Background: Because design is recognized as a critical element of engineering thinking, it is crucial for educators to utilize the most effective methods to teach engineering problem solving. Results from this study of students' thinking process may shape future teaching methods. Purpose: This article explores the differences in design processes between high school engineering students and expert engineers. It also examines the differences between high school freshmen who have taken one engineering course and seniors who have taken a series of engineering courses. Design/Method: Fifty-nine high school students from four states were asked to think aloud in a three-hour design task that was audio and video recorded. Verbal reports from the audio and video became source data for protocol analysis. Results from previous studies provided expert design performance data for comparisons. Results: Students and experts alike spent a large portion of their time modeling. Students spent significantly less time in the process of information gathering than experts. Freshmen spent significantly less time in the idea generation process than seniors and experts. Freshmen and seniors spent significantly less time determining the feasibility of their ideas, evaluating alternative ideas, and decision making than experts. Conclusions: High school students engage in design thinking with little understanding of the problem from the client's perspective. Students tend to become fixated on a single solution rather than comparing alternatives. By encouraging development of alternative solutions, K-12 engineering education could foster opportunities to critically evaluate students' design solutions.