The pivotal role of STEM education has been recognized and become a priority for countries to maintain their economic stability and improve their innovation capacity. To be able to achieve these goals, providing quality STEM education has been the ultimate goal for education systems in the two last decade. For example, the United States of America has invested hundreds of millions of dollars to increase the rigor, relevance, and content of STEM education for their K-12 students so they can continue as a world leader in all the areas they have been so far such as economy, innovation, and technology. Part of this campaign, many educational organizations in the U.S. has adopted or developed an instructional model named Project-based learning (PBL) that is engaging, rigorous, teacher-facilitated, student-centered, standards-based, and relevant. This chapter summarizes research findings on PBL; its definitions, benefits, challenges, and future research questions for the audiences of this book.