A number of theories of second language L2 acquisition acknowledge a role for explicit L2 knowledge. However, the testing of these theories remains problematic because of the lack of a widely accepted means for measuring L2 explicit knowledge. This article seeks to address this lacuna by examining L2 explicit knowledge from two perspectives. First, it considers explicit knowledge as a construct. How can explicit knowledge be defined How does it differ from other constructs such as L2 proficiency and language aptitude. Second, the article considers how L2 explicit knowledge can be measured. It critically reviews some of the ways in which explicit knowledge has been operationalized in second language acquisition research and discusses some of the instruments that have been used to measure L2 explicit knowledge. It concludes with some guidelines for investigating explicit knowledge as analyzed knowledge and as metalanguage.