A summary of the literature on relationships between climate and various types of tree diseases, and the potential effects of climate change on pathogens in western North American forests is provided. Climate change generally will lead to reductions in tree health and will improve conditions for some highly damaging pathogens. Sections on abiotic diseases, declines, canker diseases, root diseases, Phytophthoras, foliar diseases, stem rusts of pine, mistletoes, and wood decays present some examples of potential disease effects with predicted climate change. The effects of climate change on hosts, pathogens, and their interaction will have numerous, mostly adverse, consequences to forest ecosystems.