The reports on the number of microorganisms associated with seeds have increased gradually during the latter half of the 20th century. Seed is an autonomous living unit and links successive generations. Structurally, it consists of an embryo, a protective covering – seed coat, pericarp, or both – and reserve food material, which may be present in the endosperm, perisperm, or embryo. The list of saprophytic and parasitic fungi associated with seeds of different plants is very large, and they belong to all fungal classes. Seed-borne nematodes also occur as seed infestation or seed infection. Various histological techniques, e.g., embryo extraction method initially used by S. S. Skvortzov, whole-mount preparations of seed components, free hand sections, and microtome sections have been used. Microtome sections of weakly, moderately, and heavily infected seeds alone provide information on exact expanse of mycelium in seed and also the effects of host-parasite interactions. The chapter also presents an overview of the key concepts discussed in this book.