The aim of this book is to highlight the importance of plant viruses that are causing devastating plant diseases with a significant impact on agronomy and economics, focusing not only on well-known viruses but also on those that are emerging as serious threats for the production of socially important crops around the world. The book includes studies covering plant virus structures and molecular biology of virus infection/replication mechanisms, disease symptoms, host range, phylogenetic relationships, virus detection methods, available viral control strategies, plant immunity against viruses and the development of virus resistant plants, among other studies. Specifically, the book is focused on Tobacco mosaic virus, Cassava Brown Streak Viruses, Potato Virus X, blueberry and cranberry viruses, grapevine viruses, Papaya meleira virus, the causal agent of papaya sticky disease, Plum pox virus, the causative agent of sharka disease, Ageratum enation virus, a begomovirus of weeds with the potential to infect crops, begomoviruses infecting chia, Dichorhavirus associated with the citrus leprosis disease, Tomato yellow leaf curl begomovirus, Chrysanthemum stunt viroid and Chrysanthemum chlorotic mottle viroid, which infect chrysanthemums, one of the most popular flowers in the world, Emaraviruses, causing ringspot disease of European Mountain Ash, rose rosette disease, High Plains disease, raspberry leaf blotch disorder, fig mosaic disease and others.