Atypical teratoid/rhabdoid tumor (AT/RT) is a highly malignant central nervous system neoplasm that usually affects very young children and is typically deadly despite very aggressive treatment. Considered rare, the tumor was not recognized as a distinct entity until the 80's, due to its similar features with other primitive tumors. Although AT/RT has become increasingly recognized, published data has been based on small series and are retrospective. Based on these data, there are occasional long-term survivors, most of whom received intensive multi-modal therapy. AT/RT is the first pediatric brain tumor for which a candidate tumor suppressor gene has been identified. A mutation or deletion in the INI1 gene occurs in the majority of AT/RT tumors. The function of the gene is not yet understood. Prospective clinical and biologic trials are greatly needed to understand the efficacy of therapeutic interventions, as well as the role of the gene.