Extracellular proteases (proteinases, endopeptidases) are usually synthesized as larger precursors containing a signal sequence in the N-terminal part of their molecule. The signal sequence contains a sequence composed of hydrophobic amino acids which facilitate the transport of the polypeptide through membranes of the secretory apparatus. Extracellular proteinases play an important role in reproduction and spreading of tumor cells and in tissue remodelling in addition to their function in digestion food proteins. Intracellular proteinases are mostly localized in the cytoplasm and in specialized organelles (lysosomes) but are present also in nuclei and in mitochondria. They degrade or modify cell proteins and are thus involved in protein turnover which is necessary for cell viability. Intracellular proteinases are important components of the cell defense system which protects the cell against the effect of different stress factors by removing damaged and nonfunctional proteins. They modify different proteins and are thus involved in virus reproduction because some proteins are synthesized as nonfunctional virus precursors. Intracellular proteinases also play a role in the cell cycle control and in cytodifferentiation. Rather complex proteolytic enzymes were found even in very primitive organisms (archebacteria).