Mass Spectrometry is probably the fastest growing analytical technique. In the form of Liquid chromatography-tandem mass spectrometry (LC/MS/MS) it provides the preferred method for the efficient and sensitive quantitation of drugs, peptides, and ultimately the widest variety of analytes offered by any analytical technique. In addition MS often provides very sensitive and specific detection of the analyte of interest. As a result, MS-based metabolite profiling represents the dominant analytical mode. MS is evolving very rapidly as a result also of the remarkable technological advancements and large investments from the manufacturers. Mass spectrometry is taking the place of other technologies in clinical analysis aiming to substitute these in the clinical laboratory and other application fields.