Fungal hydrophobins are secreted proteins which react to interfaces between fungal cell walls and the air or between fungal cell walls and solid surfaces. They have been shown to be important in many morphogenetic processes, including sporulation, fruit body development, and infection structure formation. Hydrophobins form hydrophobic surface layers by self-assembly of secreted protein monomers in response to the environment. This process results in amphipathic polymers of interwoven rodlets on surfaces of fungal aerial structures and hyphal aggregations. Hydrophobin self-assembly is also involved in attachment of hyphae to hydrophobic surfaces and this may act as a conformational cue for certain developmental processes. Although hydrophobins appear to be ubiquitous among fungal taxa, a second class of fungal protein with very different biochemical characteristics could fulfill a similar role. These proteins, called repellents, have been identified in only one fungal species so far, but clearly help to make aerial hyphae hydrophobic. The functional similarities between hydrophobins and repellents highlight the importance of aerial development to the fungal lifestyle.