This article presents a critical review of the literature on the Second Language Motivational Self System (L2MSS) and its application in adult second language (L2) learning. By analyzing the three components of the L2MSS—the ideal L2 self, the ought-to L2 self, and the L2 learning experience, it demonstrates that for adult learners, the ideal L2 self and direct learning experiences serve as the primary motivational drivers, while the ought-to L2 self exerts a comparatively weaker influence. The paper further investigates the mediating roles of vision intervention and roadmaps in translating motivation into sustained learning behavior. It argues that vision intervention is a critical mechanism for strengthening the ideal L2 self by making it vivid and attainable, while detailed roadmaps are essential for operationalizing this vision into concrete, actionable steps. The discussion culminates in pedagogical implications, suggesting that effective adult L2 instruction should integrate systematic vision-building exercises and provide structured roadmaps to bridge the gap between motivational states and actual learning effort, thereby fostering long-term engagement and proficiency.