The expression of Wilms' tumor protein (WT1)-derived peptides on malignant cell surfaces and recognition of those peptides by cellular and humoral immune responses suggest that WT1 may be a promising potential target antigen in immunotherapeutic trials. With a high frequency of expression in hematopoietic as well as solid tumors, WT1 is a broadly applicable target. Both in vivo mouse model and in vitro human studies have demonstrated the ability of WT1-specific cytotoxic T-lymphocytes to lyse WT1-expressing malignancies without harming normal tissue. WT1-peptide vaccination, in combination with adjuvants, has demonstrated the ability to activate WT1-specific immune responses and evidence of clinical activity. Because peptide-based vaccines are human leukocyte antigen-restricted, other more broadly applicable strategies are now being developed to activate WT1-specific immune responses, including the use of WT1-specific viral vectors.