Phenotypic resistance of veterinary pathogens to more than one antimicrobial agent (multi-resistance) may be caused by intrinsic resistance to the antimicrobial agents, acquired cross-resistance, or acquired co-resistance. Known cross-resistances allow to select so-called "representative substances" which are tested and the results of which can also be regarded as being valid for other members of the same class of antimicrobial agents. In general, a limitation in the number of antimicrobial agents to be tested in routine diagnostics is necessary because of capacity and cost efficiency. This is of particular relevance when the broth microdilution method - recommended as the method of choice - with 96-well microtiter plates is used. The knowledge about the relationship between different resistance phenotypes and the corresponding resistance mechanisms is of major value for both, the laboratory personnel and the veterinary practitioner. This review explains how "representative substances" for the most relevant classes of antimicrobial agents used in veterinary medicine are chosen on the basis of known cross-resistances.