In 1994, OSHA published in the Federal Register a comprehensive proposed rule on indoor air quality (IAQ) that addressed adverse health effects attributable to environmental tobacco smoke (ETS) and other indoor pollutants, including bioaerosols (59 FR 15968). During the IAQ rulemaking, the Agency received comments and scientific and technical information on indoor mold exposures associated with building-related illnesses (BRIs). While the IAQ proposed rule was withdrawn in its entirety in December 2001 (66 FR 64946), the Agency retained the voluminous docket (consisting of approximately 120,000 documents), which contains valuable information on ETS and other indoor pollutants, such as chemicals, bacteria, and molds. In preparing this guide OSHA has reviewed the IAQ docket and recent scientific literature pertaining to mold exposures. As a result of this review, it is clear that the entrance of water (i.e., incursion) into buildings that are damaged, poorly designed, or improperly maintained, is the main source of mold-associated building-related illness. Consequently, the focus of the review was directed toward preventive measures to reduce potential environments for mold growth at the source. The purpose of this guide is to help owners, managers, and occupants understand and prevent building-related illnesses associated with mold problems in offices and other indoor workplaces. This guide provides basic information about mold, mold sources, and building-related illnesses. Brief discussions are included on building design considerations for healthy indoor air, as well as building evaluation and sampling for mold.