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.