Drug development starts with the discovery of a molecule with a therapeutic value. This can be done by high throughput screening during which separations by fast or ultra-fast HPLC are performed. At the discovery stage there can be also characterizing synthetic or natural products. Drug metabolism and pharmacokinetics (DMPK) is the step where the candidate compounds for drug are tested for their metabolism and pharmacokinetics. The studies involve use of LC-MS or LC-MS/MS. The goal in the discovery stage of drug development is to discover a new, safe and active chemical entity (NCE) that will become medication for diseases. During the last decade parallel synthesis of potential lead compounds, using combinatorial chemistry has been done. Due to its high sensitivity and selectivity, HPLC coupled with tandem mass spectrometry, HPLC-MS/MS has become the predominant method in bioassays, and pharmacokinetic and metabolic studies.