Melanization, which is brought about by the activation of the prophenoloxidase (proPO) cascade, plays an essential role in the invertebrate immune response. It is triggered by minute amounts of microbial cell wall components leading to the activation of the enzyme proPO, which converts phenols into quinones and eventually resulting to the formation of melanin. Recent studies however, revealed that the importance of proPO in invertebrate immune response, in particular towards microbial pathogens, seems to be varied. Here, we discuss the function of proPO in penaeid shrimp, a commercially important aquaculture species, and its importance to shrimp survival.