Cellular senescence, initially described as an irreversible growth arrest, has emerged as a complex and dynamic phenotype with significant implications in cancer biology and therapy. Chemotherapy-induced senescence represents a critical response to genotoxic stress, wherein cancer cells enter a state of permanent cell cycle arrest characterized by metabolic activity, morphological changes, and the secretion of pro-inflammatory factors known as the senescence-associated secretory phenotype (SASP). While chemotherapy-induced senescence serves as a barrier to tumor progression, it also contributes to genomic instability, polyploidy, and the creation of a pro-tumorigenic microenvironment, complicating its role in cancer treatment. This chapter explores the mechanisms by which chemotherapeutic agents, including doxorubicin, cisplatin, etoposide, busulfan, temozolomide, and others, induce senescence through DNA damage response (DDR) pathways involving p53, p21, and p16INK4a. These agents trigger genomic instability, leading to chromosomal aberrations and metabolic reprogramming, which can paradoxically promote tumorigenesis. The SASP, a hallmark of senescent cells, further influences the tumor microenvironment by modulating inflammation, tissue repair, and immune responses. Despite its anti-tumor effects, the persistence of senescent cells poses challenges, including therapy resistance and age-related diseases. Emerging strategies to target senescent cells, such as senolytic therapies and SASP modulation, offer promising avenues to enhance chemotherapy efficacy and mitigate adverse effects. This chapter highlights the dual role of chemotherapy-induced senescence in cancer therapy, emphasizing the need for a deeper understanding of its molecular mechanisms and the development of targeted interventions to harness its benefits while minimizing its detrimental consequences.