The cell wall is a signature organelle for a fungus. Perhaps only the nucleus contains more information than the cell wall in defining the identity, ecology and life style of a fungus. Fungi interact with and communicate with their natural environment through the cell wall interface. Their walls define their shape, impart physical strength and robustness, they define the adhesive properties of the cell, their capacity and to form biofilms and their immunological profile and other organism-specific properties. Although the wall is porous, it is also a permeability barrier to large molecules including antibiotics, antibodies and enzymes. Collectively these aspects of the cell wall largely define how a fungus establishes itself in its own niche, how it grows, obtains nutrients, causes disease and defends itself from other organisms.