SLA researchers agree that explicit knowledge and learning play an important role in adult L2 development. In the field of cognitive linguistics, it has been proposed that implicit and explicit knowledge differ in terms of their internal category structure and the processing mechanisms that operate on their representation in the human mind. It has been hypothesized that linguistic constructions which are captured easily by metalinguistic descriptions can be learned successfully through explicit processes, resulting in accurate use. However, increased accuracy of use arising from greater reliance on explicit processing may lead to decreased fluency. Taking these hypotheses as a starting point, I present a case study of an adult L2 learner whose development of oral proficiency was tracked over 17 months. Findings indicate that explicit knowledge and learning have benefits as well as limitations. Use of metalinguistic tools was associated with increased accuracy; moreover, there was no obvious trade-off between accuracy and fluency. At the same time, resource-intensive explicit processing may impose too great a cognitive load in certain circumstances, apparently resulting in implicit processes taking over. I conclude that explicit and implicit knowledge and learning should be considered together in order to gain a full understanding of L2 development. (Contains 6 figures, 1 table, and 4 notes.)