In this chapter, the authors discuss cell lines that demonstrate changes in cellular morphology, regulation of multicellular organization, responsiveness to ecdysteroids, and production of specific cellular products. They believe that these are legitimate examples of physiological and developmental capacities of insect cell lines, but each system must be examined critically. A combination of vertebrate growth factors and conditioned medium from insect cell lines leads to physiological functioning of cultured tissues, as in the case of the venom gland of the ant Pseudomyrmex triplarinus. It is fair to state that without ecdysteroid responsive cell lines, progress toward unraveling the early events associated with hormonal action in insects would have been considerably delayed. The Drosophila cell lines that were so valuable in investigating the early effects of ecdysteroid action have also been of considerable use in characterizing hormonally induced enzymatic activities. The influence of ecdysteroids on chitin synthesis and related activities has been tested in several cell lines.