Hyphae are long, branching, filamentous structures found in fungi, oomycetes, and actinobacteria. They are the main mode of vegetative growth in most fungi and are collectively called a mycelium. Hyphae are not divided into true cells, and inside them is a living protoplasm that may form tangled/large masses known as the mycelium. Each hypha is comprised of at least one cell encapsulated by a protective cell wall typically made of chitin and contains internal septa, which serve to divide the cells. Septa are important as they allow cellular organelles to pass between cells via large pores. However, not all species of fungi contain septa. The average hyphae are approximately 4 to 6 microns in size.

Hyphae may be modified in many different ways to serve specific functions. Some parasitic fungi form haustoria that function in absorption within the host cells. The arbuscules of mutualistic mycorrhizal fungi serve a similar function in nutrient exchange, so are important in assisting nutrient and water absorption by plants. Some hyphae of parasitic fungi are specialized for nutrient absorption within a specific host. These hyphae have specialized tips called haustoria, which penetrate the cell walls of plants or tissues of other organisms in order to obtain nutrients. In some fungal species, hyphae have evolved into specialized structures called sporophores, which produce spores.

In summary, hyphae are long, branching, filamentous structures found in fungi, oomycetes, and actinobacteria. They are the main mode of vegetative growth in most fungi and are collectively called a mycelium. Hyphae may be modified in many different ways to serve specific functions, such as nutrient absorption and spore production.