The human T-lymphotropic virus, also known as HTLV, refers to a group of human retroviruses which are recognized for causing a type of cancer known as adult T-cell leukemia/lymphoma, and a demyelinating disease called HTLV-1 linked myelopathy/tropical spastic paraparesis. The HTLVs are a part of larger group of primate T-lymphotropic viruses. The members of this family which infect human beings are known as HTLVs and those who infect old world monkeys are known as Simian T-lymphotropic viruses (STLVs). There are four types of HTLVs that have been found, namely, HTLV-1, HTLV-2, HTLV-3, and HTLV-4. HTLV-1 is clinically significant and it is transmitted sexually, through blood transfusion, sharing needles in drug use and breast feeding. The virus illustrates low antigenic variability and natural immunity is possible in humans. This book includes some of the vital pieces of work being conducted across the world, on various topics related to human T-lymphotropic viruses. It will help the readers in keeping pace with the rapid changes in this area of study.