Forest diseases are caused by pathogens and they affect all parts of the tree, including roots, stems, and leaves. Forest diseases result in volume losses and decreases in wood quality, which are considered detrimental in stands managed for timber products; however, in the context of a biodiversity reserve, diseases are important for nutrient cycling and habitat creation as agents of succession. This chapter uses a case study approach to describe how various forest pathogens affect trees and introduces strategies to reduce damage caused by forest pathogens in managed stands. The chapter introduces parasitic flowering plants, rust pathogens, diseases caused by Phytophthora spp., as well as abiotic disease. The chapter discusses the importance of exotic forest pathogens and the damage several of these pathogens have caused to ecosystems following their introduction, establishment, and spread. The chapter also addresses how climate change might change host–pathogen interactions, leading to differences in disease expression. Finally, the chapter provides detailed guidance on further reading.