© 2014 Elsevier Ltd All rights reserved. The cellular composition and architectural organization of the adult distal respiratory system (trachea, bronchi, and lungs) is highly complex. The respiratory system is composed of a large number of morphologically distinct cell phenotypes organized into a highly-branched series of tissues surrounding air passages. This very complex structure begins as an evagination of the undifferentiated epithelium from the foregut into a surrounding mesenchymal bundle. All of the developmental stages (embryonic, pseudoglandular, canalicular, and saccular) associated with the transformation from this simple tubular structure into a highly complex organ system are potentially susceptible to modification by toxic agents. These stages have been reviewed in more detail in earlier chapters. The susceptibility of the lung as a target organ for specific toxicants may be altered by the cellular processes involved in each of these stages of development.