Mastreviruses (family Geminiviridae, genus Mastrevirus) are distributed in tropical and temperate regions of all the continents except the Americas. The two most commonly reported mastreviruses are the type species, Maize streak virus (MSV) and Wheat dwarf virus (WDV). Both infect economically important cereals. MSV causes severe disease on maize (Zea mays L.) throughout sub-Saharan Africa and neighboring islands, and WDV affects wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.) in Europe and Asia. Infection of susceptible plant hosts leads to stunting, yellowing, chlorotic streaks on leaves, and reduced seed set, and early infection may result in total crop failure. In sub-Saharan Africa, maize streak disease is considered a significant threat to food security. In this chapter, we look at the importance of the vector for virus spread and epidemiology as well as at the interactions within the mastrevirus–leafhopper vector–plant host complex. A special focus is put on the well-studied epidemiology of WDV and interactions with MSV.