It has been well known that laboratory-acquired infections occurs in a short time period after isolation and identification of microorganisms. Working with microorganisms is a potential hazard for scientists, health care and laboratory workers, and also for the environment. The emergence of HIV, the continuing problem of HBV, and the reemergence of MT have renewed interest in biosafety. Carelessness, poor technique in the handling of infectious materials, needle-stick accidents or infectious aerosol exposure are the main cause of laboratory acquired infection. Biosafety is based on the combination of good microbiological techniques, safety equipments and facility design of the laboratory. Risk factors guideline which is the initial step in a biosafety program include the pathogenicity, the source and route of the infectious agent, worker-related risk factors, and the design of the laboratory facility. Adherence to the biosafety guidelines proposed by various governmental and accrediting agencies reduces the risk of an occupational exposure to infectious agents handled in the studies of well developed countries. Unfortunately, the importance of biosafety has not been well understood in our country. Therefore, this review examines the brief history, the causes, and the methods for prevention of laboratory-acquired infections including biosafety levels and working procedures.