Following a design-based research framework, this article reports two empirical studies with an educational MMOG, called "McLarin's Adventures," on facilitating 9th-grade students' complex problem-solving skill acquisition in interdisciplinary STEM education. The article discusses the nature of complex and ill-structured problem solving and, accordingly, how the game-based learning environment can facilitate complex problem-solving skill acquisition. The findings of the first study point to the importance of supporting cognitive regulation of students for successful complex problem-solving skill acquisition in digital game-based learning. The findings of the follow-up study show that when scaffolded by dynamic modeling, students made significant improvement in their complex problem-solving outcomes. Implications drawn from the findings of these two studies are discussed related to: (1) educational game design strategies to effectively facilitate complex problem-solving skill development; and (2) stealth or embedded assessment of progress in complex problem solving during digital game-based learning. (Contains 4 tables and 3 figures.)