In epidemiological studies, it is common to collect information on the occupations held during a person’s working life. Job coding—that is, the transcription of this information into codes for occupation (the “job”) and the sector in which that job is performed (“the sector”), using national or international classifications—allows subjects to be classified into clearly defined groups based on their occupational activity. It is then possible to study disease risk by occupation or by sector, to develop social class indicators, or to assess exposure to various hazards using job-exposure matrices (conversion tables linking occupational titles and/or sectors to specific occupational exposures inherent in their practice). Regardless of its subsequent applications, job coding is an essential step. Indeed, incorrect coding of occupations or activities leads to errors in subject classification, which can then have significant consequences for the study’s power and validity. This is why it is important to perform accurate, high-quality coding. The objective of the recommendations presented in this document is to facilitate coding, standardize it, and improve its reliability. There are numerous classifications of occupations and activities that were not originally developed for epidemiology but are nevertheless widely used in this field. The choice of classification for job coding is crucial and is based on the characteristics and capabilities of the classifications and their suitability for the study’s objectives. We will discuss here six classifications with distinct characteristics: the Classification of Occupations and Socio-Professional Categories (PCS) and the International Standard Classifications of Occupations (ISCO) 1968 and 1988 with regard to occupations, the French Classification of Economic Activities (NAF), the European Community Classification of Economic Activities (NACE), and the International Standard Industrial Classification (ISIC) for economic activities. The national system, published by INSEE, combines the PCS and the NAF, which code French occupations and economic activities, respectively. The European Community publishes the NACE, which codes European economic activities, and the UN publishes the ISIC, which codes international economic activities. The International Labour Office, for its part, develops the ISCO, the International Standard Classification of Occupations. Following an overview of data collection and coding techniques, the various nomenclatures will be reviewed, with each section including a general presentation, a proposed coding approach, specific challenges encountered, examples of frequently used codes, and their advantages and disadvantages. (R.A.)