Even parasites have parasites. By definition, a hyperparasite is an organism capable of parasitizing another parasite. Hyperparasitism caused by fungi is a common phenomenon in nature, but it has been poorly studied. This life history strategy evolved several times in the fungal tree of life, and is crucial in the maintenance of ecosystems as well as in the mediation of parasite–host interactions. Although the interest for hyperparasitic fungi is growing in the context of biological control, hyperparasitism is not ecologically and evolutionarily understood. This chapter summarizes the most relevant aspects of the terminology, diversity, and ecology of hyperparasitic fungi on both fungal and non-fungal hosts. We also discuss the problems related to molecular research on hyperparasitic fungi. As they represent a hidden source of diversity, it is necessary to increase sampling efforts and to undertake further morphological, molecular, and ecological studies to understand these fungi and their potential biotechnological and pharmaceutical uses.