The MDM2 oncogene has been suggested as a novel target for cancer therapy, based on the following observations: 1) MDM2 is overexpressed in many human cancers, including breast, colon, and prostate cancer; 2) high MDM2 levels are associated with poor prognosis in patients with cancer; 3) MDM2 overexpression is associated with advanced cancer phenotypes such as metastatic tumors and hormone-independent tumors; 4) MDM2 overexpression is associated with tumor resistance to chemotherapy and radiation therapy; and 5) inhibiting MDM2 expression or function results in tumor growth inhibition and regression. There are many options for inhibiting MDM2 function, including the use of gene silencing technologies, antibodies, peptides and small molecules. Considering the complexity of MDM2 functions, we have chosen to use gene silencing technologies including antisense oligonucleotides and RNA interference. In this article, we summarize the investigation of the antisense technology for inhibiting MDM2 expression. Antisense mixed-backbone oligonucleotides (MBO) specifically inhibit MDM2 expression in a dose- and time-dependent manner, resulting in significant anti-tumor activity in vitro and in vivo. The MBO also potentiates the therapeutic effects of chemotherapeutic agents and radiation therapy in various tumors, through both p53-dependent and p53-independent mechanisms, indicating that MDM2 inhibitors have a broad spectrum of anti-tumor activity in human cancers, regardless of p53 status. These results provide a basis for clinical evaluation of antisense anti- MDM2 oligonucleotides as chemosensitizers and radiosensitizers. In addition, the MBO has been successfully used to identify novel functions of MDM2.