This paper addresses the challenge of measuring implicit and explicit knowledge in second language (L2) acquisition. The author, Rod Ellis, from the University of Auckland, aims to establish operational definitions for these constructs and reports a psychometric study using a battery of tests. The tests include an oral imitation test, an oral narration test, a timed grammaticality judgment test (GJT), an untimed GJT, and a metalinguistic knowledge test. These tests were designed to measure implicit and explicit knowledge separately. A principal component factor analysis revealed two clear factors: one corresponding to implicit knowledge and the other to explicit knowledge. The study also discusses the theoretical underpinnings of implicit and explicit knowledge, their relationship, and the methods used to distinguish between them. The results support the validity of the tests in measuring the two types of knowledge, providing a foundation for further research in L2 acquisition.This paper addresses the challenge of measuring implicit and explicit knowledge in second language (L2) acquisition. The author, Rod Ellis, from the University of Auckland, aims to establish operational definitions for these constructs and reports a psychometric study using a battery of tests. The tests include an oral imitation test, an oral narration test, a timed grammaticality judgment test (GJT), an untimed GJT, and a metalinguistic knowledge test. These tests were designed to measure implicit and explicit knowledge separately. A principal component factor analysis revealed two clear factors: one corresponding to implicit knowledge and the other to explicit knowledge. The study also discusses the theoretical underpinnings of implicit and explicit knowledge, their relationship, and the methods used to distinguish between them. The results support the validity of the tests in measuring the two types of knowledge, providing a foundation for further research in L2 acquisition.