Self-assessment practices have become widespread in second language (L2) education, and while its proponents have long argued that such reflection enhances learner awareness of their abilities and promotes independent learning (Alderson, 2005; Chen, 2008; Little, 2007), others have questioned its value vis-a-vis other indicators of performance (Gipps, 1994; Ross, 1998). This article approaches learner self-assessment from a Vygotskian perspective, and specifically the proposal of the Zone of Proximal Development (ZPD). According to this view, development is understood as transitioning from other-regulated to self-regulated functioning. Following Zuckerman (2003), engaging in "reflective evaluation" of performance is argued to be a critical feature of this process. Data from a study involving L2 learners of French participating in a Dynamic Assessment (Lantolf & Poehner, 2004) program are presented to explore how this development may occur and to examine challenges learners may experience as they endeavor to regulate not only their use of the L2 during completion of tasks but also while evaluating their performance. In keeping with the notion of the ZPD, it is argued that learner efforts to self-assess must be carefully mediated as they move from a "cooperative" to an "independent" mode of self-assessment. (Contains 2 notes.)