Biofilms are microbial communities that grow attached to a surface and are enclosed in a matrix of extracellular polymeric substances. These microbial communities comprise a single or diverse species, which occupy the same functional discrete environment. Despite resistance to accept and define filamentous fungi (ff) biofilms, nowadays these microorganisms are well known by their capability to form biofilms. Filamentous fungi follow the proposed criteria for biofilm formation, that is present structural features such as complex aggregated growth, surface-associated growth of cells and secreted extracellular polymeric matrix and altered gene expression resulting in phenotypic changes that include enhanced tolerance to antimicrobial compounds or biocides, changes in enzyme or metabolite production and/or secretion physiological changes. Extensive studies have been carried out with Aspergillus niger and Aspergillus fumigatus biofilms, confirming the production of an extracellular polymeric matrix, differential gene expression and differential sensitivity to anti-fungal drugs. Hydrophobicity is reported as an important feature that influences ff biofilm formation, that is the development of differentiated mycelial zone, which can be associated with further interactions in aquatic environments. On the other hand, few researches include studies about morphological and physiological characteristics of ff biofilms. This chapter also presents a summary of research fields and techniques applied in biofilm studies that may help in further studies in the area.