The purpose of this paper is to overview research efforts at the NTCIR-6 CLIR task, which is a project of large-scale retrieval experiments on cross-lingual information retrieval (CLIR) of Chinese, Japanese, Korean, and English. The project has three sub-tasks, multi-lingual IR (MLIR), bilingual IR (BLIR), and single language IR (SLIR), in which many research groups from ten countries or regions are participating. This paper describes the system of the NTCIR-6 CLIR task and its test collection (document sets, topic sets, and method for relevance judgments), and reviews CLIR techniques used by participants and search performance of runs submitted for evaluation.