Stereoisomers are compounds that have identical sets of atoms configured in the same positions but are arranged differently spatially. Approximately 25% of contemporary drugs are marketed and used as racemates (i.e., as equimolar mixtures of stereoisomers). This may have major clinical implications, as stereoisomers may possess qualitative and/or quantitative differences in pharmacological effects, plasma protein and tissue binding, metabolic and renal clearance. There are many examples of racemic drugs manufactured and used as single stereoisomers in the field of neurology including the anti-Parkinsonian drugs levodopa, selegiline, apomorphine and entacapone, the antiepileptic drugs tiagabine and levetiracetam, the secondary stroke prevention agent clopidogrel and the acetylcholinesterase inhibitor rivastigmine. The role of drug stereochemistry in the re-evaluation of established drugs and the production of new agents is becoming increasingly important as pharmaceutical companies endeavour to show proof of "no penalty" for the introduction of a racemic new drug over one or other of its single stereoisomers.