This book provides a comprehensive review of the emerging role of extrachromosomal DNA (ecDNA) in cancer, highlighting its significance in tumor progression, oncogene amplification, drug resistance, and clinical applications. It explores the structural and functional characteristics of ecDNA, emphasizing its role in shaping the genetic and epigenetic landscapes of cancer cells. The book discusses the impact of ecDNA on tumor heterogeneity, metastasis, and immune evasion, providing insights into how ecDNA contributes to cancer evolution and therapeutic resistance. The book also examines cutting-edge imaging techniques, computational approaches, and non-invasive diagnostic applications of ecDNA, highlighting its potential as a biomarker in precision oncology. Additionally, it presents the novel therapeutic strategies targeting ecDNA, addressing challenges and opportunities in integrating these insights into cancer treatment. 1) Explores the structural formation and oncogenic role of ecDNA in tumor development and progression.2) Discusses the influence of ecDNA on gene expression, epigenetics, and immune modulation in cancer. 3) Examines the role of ecDNA in drug resistance, metastasis, and disease recurrence. 4) Highlights advanced imaging, computational tools, and non-invasive diagnostic applications of ecDNA.5) Reviews emerging therapeutic strategies targeting ecDNA for improved cancer treatment and personalized medicine. This book is intended for researchers and students of cancer biology, molecular biology, and biotechnology.