Most of fatty acid double bonds are in cis configuration, but some processes may isomerise double bonds from cis to trans configuration and from their naturally occurring positions in the carbon chain. Since some fatty acids with trans double bond(s) are suspected to have adverse effects, several countries have regulated the inclusion of trans fatty acid (TFA) content labelling. The French governmental authorities requested AFSSA to propose recommendations regarding food labelling. This first article summarises the discussion on a regulatory TFA definition and reviews the occurrence and origins of TFA in foods. From a chemical point of view, a TFA is any kind of fatty acid that has one or more double bond(s) in trans configuration, but restricted TFA definitions have been proposed for food labelling purposes. These definitions exclude fatty acids with conjugated double bonds on different criteria which are commented on in this paper. Taking into account the goal of TFA labelling and the potential harmful properties of some trans conjugated fatty acids, the working group proposes the chemical definition for TFA labelling purposes. The main TFA natural origin is ruminal biohydrogenation, and therefore TFA are present in dairy products and ruminant meat. TFA are also found in domestically or industrially processed foods due to hydrogenation or heat treatment effects on oils and fats. Although the TFA molecules produced by both natural and industrial processes are often quite similar, TFA distribution profiles and levels are very different from one case to another.