Tumor cells have a remarkable ability to survive in harsh circumstances, resist treatment and evade regulatory mechanisms like apoptosis and therapy. They alter their metabolism in order to sustain metastatic progression and uncontrolled growth. Functional and phenotypic heterogeneity in tumor cells gives them metastatic potential, aggressiveness and the capability to resist treatment. This heterogeneity is governed by a range of intrinsic and extrinsic stimuli involving the ones from the tumor microenvironment. Various tumor cells can exhibit heterogeneous phenotypic and morphological profiles comprising gene expression, metastatic potential, metabolism, proliferation, cellular morphology and motility. It have been detected in many types of cancer including breast cancer, head and neck cancer, gynecological carcinoma, multiple myeloma, leukemia, prostate cancer, colon cancer, bladder, brain cancer and liposarcoma. This book contains some path-breaking studies on tumor heterogeneity. It strives to provide a fair idea about tumor heterogeneity and to help develop a better understanding of the diagnostics and therapeutic management of tumors. This book will help new researchers by foregrounding their knowledge in this area of study.