People need to detect biocontrol agents and genetically engineered microorganisms, both to study the ecology of these introduced organisms in the environment and for risk assessment. They may also want to monitor a biocontrol product in the environment, and the ability to do this may even be required in the not too distant future, for example, to comply with regulations. This chapter describes methods for the detection of bacteria. The bacterial strain of choice is marked by selecting a derivative that is resistant to an antibiotic that can be added to solidified agar media. Natural antibiotic resistance can also be used for detection of some organisms. The use of dilution plating techniques on the particular selective medium then allows enumeration of microbial populations. In the colony hybridization method, only culturable cells are detected, and when a nonselective medium is used, the usefulness of this method may be limited where there is a low frequency of positive colonies.