Bioanalytical method development is the process of creating a procedure to enable a chemical entity of potential therapeutic value to be quantied in a biological matrix. This entity may be either an active principle, a metabolite, or surrogate marker. Such methods are required to demonstrate exposure of the test system to the compound for toxicological assessment, as an aid to understanding its pharmacokinetic and pharmacodynamic behaviour, or in the understanding of surrogate end points in the case of biomarkers. This chapter outlines the major steps that may be applied to establish a new quantitative bioanalytical method and is intended to be read together with other chapters that deal with each of the aspects in more detail.