Second language acquisition research over the past three decades has generated a wide spectrum of different interpretations of “fossilization”– a construct introduced by Selinker (1972) for characterizing lack of grammatical development in second language learning. These conceptual differences found in the literature, it has become increasingly clear, create confusion rather than offering clarification, thereby obstructing a coherent understanding of the theoretical notion as well as empirical research findings. This article addresses the conceptual differences by raising and discussing five central issues: (1) Is fossilization global or local? (2) Is L2 ultimate attainment isomorphic with fossilization? (3) Is fossilization a product or a process? (4) Is stabilization synonymous with fossilization? (5) Should empirical studies of fossilization span five years or more?