Bioanalytical data are an essential and integrated part of the safety evaluation of new drugs. This chapter discusses regulated bioanalysis and guidelines in detail. It discusses the historical evolution of regulated bioanalysis, bioanalytical method validation (BMV), pre-study validation, in-study validation, documentation, regulatory requirements, and quality systems in regulated bioanalysis. The basic parameters for a BMV are accuracy, precision, selectivity, sensitivity, reproducibility, and stability.The process of developing a new drug from drug discovery to drug development typically covers a period of more than 10 years. Bioanalytical support to studies that are part of a drug application file, conducted by guidance from dedicated regulatory bioanalytical guidelines and often conducted under GLP or Good Clinical Practice (GCP), is referred to as regulated bioanalysis. Bioanalysis was becoming hyphenated, for example liquid chromatography–tandem mass spectrometry (LC-MS/MS), with the advantages of increased sensitivity and selectivity.