The formation of a biofilm assists microbes in survival in their environments, defends against stresses, and may enable the extension of its range. In this chapter, we review various aspects of the biofilms of Aspergillus, an important ubiquitous global fungus, found in many environments, and capable of causing diseases, decay and/or intoxications. The extracellular matrix is a very important component of these biofilms. Biofilm formation is affected by the environmental situation and affects the biology and physiology of the fungus. Here, we review the course of Aspergillus biofilm formation, its composition, its architecture, the interactions with the hosts (particularly patients), in intermicrobial competition, in nature, and its effects in the industry. Mixed biofilms with other microbes are explored, as is the interaction with antifungal molecules. All the features just enumerated and reviewed have implications for understanding fungal biology and the development of treatment strategies.