Disease is an important factor in reducing shrimp numbers in natural populations. Natural mortality or disease from old age is the potential fate of all shrimp, but predation, starvation, infestation, infection, and adverse environmental conditions are highly significant. Information on the life cycles and general biological characteristics of common parasites and commensals of commercial penaeid shrimp are discussed. Gregarines, microsporidians, and ectocommensal protozoa are grouped as protozoan parasites and have both internal and external effects upon shrimp. Although worms have not been a high factor in shrimp mortality, they have been found in various parts of the body. Trematodes, cestodes, and nematodes are among the most common worms found.