In general, proteases are involved in diverse functions; the most notable include digestive, protective, and regulatory processes. Digestive proteases are involved in protein degradation for nutritional purposes. About 35% of all entries in the MEROPS database are classified as serine proteases. Two other periplasmic serine proteases containing PDZ domains, DegP and DegQ, are also part of this family. Buchanan and Sowell recognized a considerable rise in expression level of PBP6 in stationary- phase cells compared with exponentially growing cells . While sbmC is a stationary-phase-induced E. coli SOS gene involved in MccB17 susceptibility, sbcB encodes for exonuclease I. PBP6b has a molecular mass of 43 kDa and was classified as belonging to the family S11 (clan SE) because it shares several molecular features of this group. The 50 families and 16 clans of metalloproteases recognized to date indicate that they are the most diverse of the four main types of proteases. For the peptidases discussed in this chapter there are two relevant mechanisms of catalysis. The first mechanism involves the catalytic His His Glu/Asp sites for zinc binding. Structural and kinetic studies allow proposing the following mechanism for CPD A. This chapter provides a summary of the information available for pitrilysin, MepA, alkaline phosphatase iso-zyme conversion protein, YebA, YfgC, and YhjT. The chapter talks about cysteine proteases, Proteases of unknown classification, and protease inhibitor.