Hazardous atmospheric conditions threatening worker safety within confined spaces are reviewed. As defined by NIOSH, a confined space has limited openings, unfavorable natural ventilation, and is not designed for continuous occupancy. Worker entry is often necessary for reasons such as repair or maintenance. A NIOSH review indicated that fatalities have resulted from a high percentage of confined space accidents. Of 276 events studied, 80 were caused by atmospheric conditions, resulting in 78 deaths and 72 injuries. Asphyxiating atmospheres occur in confined spaces when oxygen is reduced. Flammable atmospheres occur when flammable gases, vapors, or dusts accumulate, particularly in the presence of oxygen. Toxic atmospheres and irritants or corrosive atmospheres are frequently generated. Case reports are given for each of these atmosphere types. NIOSH guidelines call for delaying entry into confined spaces until atmospheric testing is completed. Parameters used to classify confined space hazards are characteristics of the space, oxygen content, flammability, and toxicity. Three categories of confined space are defined. A check list of considerations for entry or working in confined spaces is presented. The author concludes that proper precautionary measures can only be taken when known and potential hazards of confined spaces are identified before worker entry.