Tumor suppressor gene (TSG) replacement therapy that involves various delivery systems is emerging as a promising antitumor strategy because malignant tumors develop through genetic alterations in TSGs. The most potent therapeutic TSG for tumor suppression is the multifunctional transcription factor p53 gene that regulates diverse cellular phenomena such as cell cycle arrest, senescence, apoptosis, and autophagy. Since the p53 gene is frequently inactivated by aberrant genetic regulation in human cancers, p53 replacement therapy is widely and frequently used as a potent antitumor strategy to restore wild-type p53 function in the p53-inactivated tumors. This chapter focuses on four types of p53 transfer systems: cationic liposome-DNA plasmid complexes, a replication-deficient adenovirus vector, a replication-competent adenovirus vector, and a protein transduction system. Moreover, we discuss recent advances in our understanding of the molecular basis of the p53-mediated cell death signaling pathway and therapeutic methods for enhancing tumor cell death and induction of bystander effects within tumor tissues in p53 replacement therapy. Exploration of the molecular mechanism underlying the p53-mediated tumor-suppressive network system and development of an effective strategy for enhancing p53-mediated cell death signaling pathways would lead to an improvement in the clinical outcome of patients with p53-inactivated cancers.