Summary Microorganisms are ubiquitous in our environment: they are present in water, soil, air, plants, animals and people. In the workplace, the interest in microorganisms mainly relates to their presence in the air; they are then called bioaerosols. Bioaerosols are defined as airborne particles consisting of living organisms such as microorganisms or originating from living organisms, such as metabolites, toxins or fragments of microorganisms. For the majority of them, the dose/effect relationships by inhalation have not been established, but the scientific community nevertheless agrees that some bioaerosols may cause health problems. The international interest in bioaerosols as an agent that can affect workplace air quality and workers’ health has rapidly increased the pool of knowledge on their identification, quantification, their presence in different workplaces, and the effects that they can produce in the people exposed to them. Different approaches for risk evaluation are used by occupational health researchers and professionals, which raises questions mainly about the types of microorganisms or derivatives to be investigated; the purpose, techniques and locations of sampling; result interpretation, taking into account the lack of exposure standards and dose/effect relationships, and the most effective means for correcting an abnormal situation or for maintaining healthy conditions. This practical guide describes the approach recommended by the IRSST for the evaluation, control and prevention of bioaerosol exposure; it corresponds to the typical industrial hygiene procedure, namely the anticipation, identification and evaluation of the risks, with the ultimatepurpose of controlling exposures in order to prevent disease.