This guide provides methodological guidance for implementing human biomonitoring (BM) following a technological accident. BM is valuable for characterizing exposure and health effects in the population following a technological accident. Identifying chemical agents and situations that may warrant early BM, as well as determining the specific procedures for such monitoring, are tasks for experts. To this end, a characterization of chemical emissions resulting from the technological accident is necessary. This preliminary step may take several days, and waiting for the results before deciding to collect samples would make it impossible to conduct early BI (i.e., within the first 48 hours after emissions cease). This situation could be detrimental to the assessment of exposures and associated health risks, as it would preclude any possibility of evaluating exposures to chemical agents with rapid elimination kinetics. It is therefore proposed that an initial phase of biomonitoring be conducted no later than 48 hours after the end of emissions from the accident. Given the tight deadlines for this first phase, it is recommended to target those most exposed and most affected by the accident, that is, all individuals (workers, occupants, responders, etc.) who were present at the site of the industrial accident, as well as all individuals seeking care at a healthcare facility, from the day of the accident until 48 hours after the end of emissions from the industrial accident, for a reason related to the accident. Following this initial phase, medium- and long-term biological monitoring may be considered based on available information regarding the released substances and the observed environmental contamination. During this second phase, particular attention should be paid to populations at high risk of exposure and to sensitive populations. The guide details the practical procedures for implementing biological monitoring and interpreting its results, including, in particular, the questionnaire accompanying the collection of biological samples; the organization of the collection, transport, and storage of emergency samples; the criteria for selecting the analytical laboratory; and the principles for interpreting the results of biological exposure indicator assays. A technological accident may cause significant stress among the public and those directly involved; therefore, the steps for communicating and reporting the results of the BI are crucial from the beginning to the end of the process. This guide also provides information regarding this communication and engagement with the public, including a list of frequently asked questions about the BI and the expected health effects. A methodology for the individual and collective reporting of SB results is also proposed. Finally, this guide emphasizes the importance of complying with regulatory and ethical requirements, as well as the recording and retention of information.