This article explores the relationship between cellular turgor pressure and mechanisms used by fungi to invade solid substrates. In the oomycete Saprolegnia ferax, the rate of hyphal growth through solid medium decreases as turgor is reduced and the effect is most pronounced at high agar concentrations. This is the first clear evidence that turgor provides the force for invasive hyphal growth. Among pathogenic fungi, the role of turgor in plant infection has been established by experiments on the rice blast fungus Magnaporthe grisea, which can punch through the surface of rice leaves and a variety of synthetic membranes. In common with other walled organisms, fungal growth hinges on an interplay between turgor and the resistance offered by the wall; irrespective of turgor, no cellular expansion or substrate deformation can occur unless the wall yields. Turgor is the only logical source of the necessary force when fungi penetrate plastics, lift the lids of Petri dishes, or burst through asphalt paving. In other cases, fungi use exoenzymes to soften the substrate in advance of the invading cells. This process is particularly significant in plant infection, which involves a combination of physical force and the secretion of cuticle- and wall-degrading enzymes. Key words: hyphae, osmotic stress, Oomycetes, Magnaporthe, Saprolegnia, turgor pressure.