For each pesticide used in food or feed productions, the legislative authorities have to establish the residue definitions, that mean compounds which need to be taken into account in risk assessment in process of authorization of plant protection products as well as in establishing and enforcing Maximum Residue Levels (MRLs). The residue definition for a given pesticide can be different depending on the aim for which they are used. Inclusion of a compound in such definition results in consequence in its later analysis in different matrices during monitoring and official control. Inclusion of compounds in the definition depends on a number of factors, and the decisions on whether they should be included have to be made for every case separately. The definition should include toxicologically important compounds, and those which contribute into the residue in 10 or more percent of TRR, or those that have been detected in livestock and plants as metabolites different than those found in rats.