Bioanalysis is an essential part in drug discovery and development. Bioanalysis is related to the analysis of analytes (drugs, metabolites, biomarkers) in biological samples and it involves several steps from sample collection to sample analysis and data reporting. The first step is sample collection from clinical or preclinical studies; then sending the samples to laboratory for analysis. Second step is sample clean-up (sample preparation) and it is very important step in bioanalysis. In order to reach reliable results, a robust and stable sample preparation method should be applied. The role of sample preparation is to remove interferences from sample matrix and improve analytical system performance. Sample preparation is often labor intensive and timeconsuming. Last step is the sample analysis and detection. For separation and detection, liquid chromatography-tandem mass spectrometry (LC MS/MS) is method of choice in bioanalytical laboratories. This is due to high selectivity and high sensitivity of the LC MS/MS technique. In addition the information about the analyte chemical structure and chemical properties is important to be known before the start of bioanalytical work. This review provides an overview of bioanalytical method development and validation. The main principles of method validation will be discussed. In this review GLP and regulated bioanalysis are described. Commonly used sample preparation techniques will be presented. In addition the role of LC MS/MS in modern bioanalysis will be discussed. In the present review we have our focus on bioanalysis of small molecules.