Taxonomy is one of the most important basic specialties in any biological control project. Proper identification of the target pest and associated organisms can greatly influence the final outcome of any introduction or manipulation of natural enemies. The determination of population characteristics, not morphologically identifiable, is frequently necessary in order to successfully establish a certain species of natural enemy in a new region. Several recent examples have indicated that different but morphologically very similar (sibling) species as well as biotypes of a same species behave differently in a new environment, in accordance with adaptations undergone in response to differential local stresses in their original range of distribution. A number ofprojects conducted worldwide has indicated the need for taxonomists to go beyond the morphological considerations if their work is to be best applied to biological control.