The development of inhalation toxicology as a distinct discipline can be traced back well over one hundred years. The technology has advanced in terms of materials and designs used to construct inhalation chambers and the equipment used to generate controlled test atmospheres ofa wide variety of gases, vapors, dusts, and droplets. Consideration of metered dose inhalers, a relatively recent concern, has led to the design of new equipment for administering this unique dosage form. The parameters used to evaluate inhalation toxicity are similar to those used for any other route of administration. In addition, there are some unique procedures for early screening of pulmonary toxicity, especially within a series of related chemicals.