Improvement of rates of colonization is essential to enhance effectiveness of biological control programs based on natural enemy introductions. Under ideal conditions, agents introduced should be species and strains well adapted to the target pest and able to attack it on the target crop plant. Agents should be chosen, if feasible, from sources climatically similar to intended release areas. Release areas should meet the diapause needs of agents. Release sites should be physically secure, stable, have abundant target hosts, and be well located for further spread of the agent to new areas. Numbers, quality, and stages of agents should be selected so that adequate numbers of healthy, mated (if adults), fed agents are released at colonization sites. Agents may be released in various life stages including adults, immatures, and parasitized hosts. Releases must be timed so that the life histories of the agent and target pest are appropriately synchronized. Cages and mechanical release systems must be chosen with the agent and pest biology in mind. Colonization efforts should be sustained and revised as needed until colonization is achieved or the agent is clearly shown to be biologically unable to establish. In practice, some deviation from these ideal conditions is required to prevent the loss of valuable biological material which is available at the moment.