The chapter presents an overview of safety tests of microbial insecticides to estuarine and marine organisms that have been performed to date. Approaches and experimental design, species of MPCAs tested, systems used, and endpoints and results evaluated for determination of risks of MPCAs to nontarget marine species are reviewed. The review order is by exemplary microbial agents, as follows: viruses, bacteria, fungi and protozoa. The studies presented describe relatively simple procedures for exposing single species nontarget hosts to MPCAs. They incorporate a positive control bioassay to confirm the infectivity of the MPCA and are relatively inexpensive and reliable.