Rational drug discovery requires an early appraisal of all factors impacting on the likely success of a drug candidate in the subsequent preclinical, clinical and commercial phases of drug development. The study of absorption, distribution, metabolism, excretion and pharmacokinetics (ADME/PK) has developed into a relatively mature discipline in drug discovery through the application of well-established in vitro and in vivo methodologies. The availability of improved analytical and automation technologies has dramatically increased our ability to dissect out the fundamentals of ADME/PK through the development of increasingly powerful in silico methods. This is fuelling a shift away from the traditional, empirical nature of ADME/PK towards a more rational, in cerebro approach to drug design.