A virus is an infectious agent that can multiply inside the living cells of an organism. The scientific discipline concerned with the study of virus is called virology. This discipline is focused on the structure, interactions, ecology, evolution, physiology and clinical aspects of viruses. Virus classification refers to the process of identifying and classifying viruses into a taxonomic system. They are majorly classified into animal virus, plant virus, etc. Virology also studies subviral particles like viroids and satellites, which are smaller and simpler than viruses. Viral diseases are a major focus area under virology. Viral disease or infection occurs when an organism's body is infected by pathogenic and infectious viruses and related particles. Such diseases are common cold, HIV/AIDS, parainfluenza virus infection, etc. This book studies, analyzes and upholds the pillars of virology and its utmost significance in modern times. Different approaches, evaluations, methodologies and studies in this field have been included in this book. The topics covered in this textbook offer the readers new insights in the field of virology.