A workshop on childhood atypical teratoid/rhabdoid tumors of the central nervous system, sponsored by the National Cancer Institute and the Pediatric Brain Tumor Foundation of the United States, was held on January 29, 2001 in Bethesda, Maryland. Drs. Malcolm Smith, Jaclyn Biegel, and Roger Packer hosted the meeting. There were 22 participants from 14 institutions. The sessions were designed to review what was known about the biology and clinical behavior of rhabdoid tumors and to begin to develop treatment strategies that might be effective against this clinically aggressive group of malignancies. This report summarizes the biology sessions from the meeting. A detailed summary of the discussions regarding diagnostic studies and the roles for radiation therapy, chemotherapy, and bone marrow or stem cell transplant in the treatment of rhabdoid tumors will be published separately.