True fungi are a diverse and widespread group of microorganisms classified in the kingdom Eumycota. Fungi are eukaryotic, heterotrophic microorganisms. Most fungi are microscopic, but some have large fruiting bodies such as hat mushrooms and bracket fungi. Most of them are aerobic, but a few are anaerobic. They either are single-celled (yeasts) or grow as thread-like structures (hyphae) to form a mycelium. Fungi reproduce primarily by means of spores. Fungi constitute the largest group of organisms causing plant diseases. Most fungi are, however, saprophytic and are very important for the decomposition of organic material. This chapter will give an overview of the characteristics and classification of plant pathogenic fungi, and describe some species causing important plant diseases.