The development and validation of quantitative bioanalytical methods to measure drug and biotransformation products (metabolites) in biological matrices has been evolving for decades. The uninterrupted introduction of new technologies and the increasing attention being paid by global regulatory authorities to validation issues has continued to shape this scientific field. However, in the last five years, there has been an unprecedented increase both in the pace and breadth, of research activities related to drug discovery and development.’ The high-throughput screening of compound libraries has dramatically increased the number of potential drug candidates in discovery programs. The resulting need to evaluate these candidates during lead optimization has fueled extraordinary growth in the number of samples being analyzed in bioanalytical laboratories for the quantitative determination of both drugs and putative metabolites in biological matrices such as blood, plasma, serum and urine. Additionally, significantly greater attention is being focused on the metabolic liabilities of these compounds at earlier stages in drug development than ever before. In a continuing effort to keep up with the increasing demands for higher sample throughput, greater sensitivity, and increased metabolic information, bioanalytical scientists are continually accelerating their efforts to search for technological advances. This pattern of change began three decades ago and continues today.