Although it is well known that many second language (L2) learners have trouble using articles “properly,” the primary causes of their difficulties remain unclear. This study addresses this problem by examining the metalinguistic knowledge of the English article system that learners employ when selecting articles in a given situation. By doing this, the present study attempts to better understand the process of “making sense” of the English article system by learners who are at different stages in their interlanguage development. Eighty Japanese college students with varying levels of English proficiency participated in this study. Immediately after completing a fill-in-thearticle test, a structured interview was conducted to investigate the reasons for their article choices. The quantitative and qualitative analyses reveal a number of conceptual differences with regard to their considerations of the hearer’s knowledge, specific reference, and countability, which may account for learners’ errors in article use across different proficiency groups.