Many different bacteria secrete proteases into the culture medium. Extracellular proteases produced by Gram-positive bacteria are secreted by a signal-peptide-dependent pathway and have a propeptide located between the signal peptide and the mature protein. Many extracellular proteases synthesized by Gram-negative bacteria are also produced as precursors with a signal peptide. However, at least two species of Gram-negative bacteria secrete one or more proteases via a novel signal-peptide-independent route. Most proteases secreted by Gram-negative bacteria also have a propeptide whose length and location vary according to the protease. Specific features of protease secretion pathways and the mechanisms of protease activation are discussed with particular reference to some of the best-characterized extracellular proteases produced by Gram-positive and Gram-negative bacteria.