The coherent body of research described in this book is concerned with new HPLC method development and validation using novel systematic approaches for pharmaceutical and diagnostic compounds. The first stage of the research was to study how analytical method development and validation are typically carried out at present and to formulate this into a simple step-by-step approach. Such a template and protocol was not only used as the foundation of this research programme but could also serve as a simple systematic guide for other practitioners and those new to the field. Furthermore, it was recognised that this protocol should satisfy the requirements of the most strategically important regulatory agencies. The second stage of this research involved evaluation and application of the above validation approach to new methods that were developed for a diverse range of analytes using HPLC and LC-MS. In essence, the critical review of the requirements for method validation for various agencies and the subsequent preparation of guidelines on how to go about method validation have had a significant impact on analytical practitioners worldwide.