This chapter describes various airborne viable microbial allergens. It discusses the means for growth and amplification, and explains sampling methods. Some methods may be better than others wherein the intent and information sought in the various circumstances are highly variable. There have been a few attempts by professional organizations, universities, and private firms to provide guidelines. In 1986, the American Conference of Governmental Industrial Hygienists (ACGIH) published a report with the intent to provide a guideline to industrial hygienists for evaluating "workplace-related illness such as hypersensitivity pneumonitis, humidifier fever, and allergies that are likely due to bioaerosols". They later published a more expansive variation with considerable changes in interpretation. These details and a discussion thereof may contribute "food for thought". While comparison sampling is a sound scientific approach for ascertaining amplification, viable sampling provides only a piece of the picture.