In recent years, researchers have become increasingly interested in the notion of spontaneous focus on form (FonF) (i.e., attention to linguistic forms that arise incidentally during meaningful communication). Previous research has indicated that such FonF is beneficial for L2 learning. However, this research has focused mainly on reactive FonF, with relatively little investigation into pre-emptive FonF. This paper presents findings from a large-scale, classroom-based study that examined the role of both reactive and pre-emptive FonF in second-language development as measured by tailor-made, student-specific post-tests. Data from 54 hours of classroom interaction within seven intact ESL classes at three levels of language proficiency revealed that both reactive and pre-emptive FonF occurred. However, pre-emptive FonF led to higher individualized post-test scores than did reactive FonF. Furthermore, the amount, type, and effectiveness of FonF were strongly related to the learners' level of language proficiency. (Contains 4 notes and 8 tables.)