This chapter establishes a catalog of the proteases secreted by Aspergillus fumigatus, briefly describes their various properties, and examines their biological functions, ranging from protein digestion into short peptides and assimilable amino acids to specific proteolysis during infection. The proteases include all enzymes that catalyze the cleavage of peptide bonds (CO-NH) of proteins, digesting them into peptides or free amino acids. The proteases can be further divided into endoproteases and exoproteases. From a physiological point of view, Aspergillus secreted proteases can be classified as either acidic, neutral, or basic. Efficient protein degradation of hard keratin by hydrolytic enzymes has to be accompanied by simultaneous reduction of cysteine disulfide bridges. Although it can be stated that individual proteases such as Alp1, Mep, and Pep1 are not essential for tissue invasion, it cannot be ruled out that secreted proteases do not contribute to the establishment of invasive aspergillosis. A. fumigatus also does not possess specific large gene families encoding secreted proteases. Aspartic proteases of the A1 family and carboxypeptidases of the S10 family are multiple in many fungi. The emergence of multigenic families is most frequently due to ancient gene duplication processes allowing organisms to better adapt to different environmental conditions, and marked differences occur from one pathogenic species to another.