Veterinary drugs may only be produced, distributed and administered after being licensed. This implies that, prior to marketing, a critical evaluation of the pharmaceutical quality, the clinical efficacy and the over-all pharmacological and toxicological properties of the active substances will be performed by national and/or supranational authorities. However, despite a sophisticated legal (harmonized) framework, a number of factors involved in residue formation and safety assessment remain unpredictable or dependant on the current 'state of the art' in the understanding of molecular pharmacology and toxicology. For example, drug disposition and residue formation in the target animal species may be influenced by a broad variety of physiological parameters including age, sex and diet, as well as by pathological conditions especially the acute phase response to infection. These factors affect both drug disposition and metabolite formation. Furthermore, current thinking in toxicological risk assessment is influenced by recent developments in molecular toxicology and thus by an increased but still incomplete understanding of the interaction of a toxic compound with the living organism. General recognized principles in the evaluation of potential toxicants are applied in the recommendation of withdrawal times and the establishment of maximum residue limits (MRL values). Apart from toxicological-based assessment, increasing awareness is directed to other than toxicological responses, especially the potential risk of effects of antimicrobial residues on human gastrointestinal microflora. Thus, the methodology of risk assessment is discussed in the context of the recently established legal framework within the European Union.