There are many ways in which different species of fungi can interact with each other. At one extreme, one living fungus serves directly as the nutrient source for another. This parasitic relationship, often termed “mycoparasitism” is the main focus of this book, particularly with respect to necrotrophic and biotrophic associations. However, fungicolous fungi, which have a constant but indeterminate interfungal association and hence may be parasitic, are also considered. Substantial chapters review physiological and ecological aspects of mycoparasitism while the final chapter examines biological control by fungi of fungal plant pathogens.