Background: Developing self-directed learning (SDL) skills in engineering students is critical to support life-long learning. This research was conducted at Iron Range Engineering (IRE), an innovative, problem-based learning (PBL) engineering program, which has suffused the concept of metacognition throughout its curriculum. Purpose/Hypotheses: (1) Using measures of metacognition obtained from think-aloud problem-solving activities, to what extent do students change their use of SDL strategies from the beginning of their junior year to the end of their senior year of the 2-year PBL engineering program?; (2) Using evidence obtained from interview data, how do students' conceptualizations of metacognition and use of SDL strategies change from the beginning of their junior year to the end of their senior year of the 2-year PBL engineering program?; and (3) What is the relationship between think-aloud and interview measures of metacognition, and how could the two types of measures better inform us about the potential impact of the IRE program on students' use of SDL strategies? Design/Method: Twelve students participated in both think-aloud problem-solving tasks and interviews. Results: Data from think-aloud problem-solving activities showed that nearly one-half of students' verbalizations were metacognitive and that they talked more about strategies and made more use of domain knowledge by the end of their program. Interview data showed that one-half of the students showed an increase in the complexity of their understanding of SDL. Conclusions: Our data show an increased use of strategies during problem-solving at the end of the program. The results are promising, but further research is needed to understand the development of SDL in engineering students.