The aim of medical treatment is to achieve optimal drug response and avoid harmful side effects for the individual patient. Clinical Pharmacology is the study of pharmacokinetic processes including absorption, distribution, metabolism and excretion, and the response to drugs in the human body. Individual variability in both the pharmacokinetics and effects of drugs is common. The effects/side effects of many drugs are concentration-dependent, highlighting the usefulness of total drug concentration measurement in serum/plasma, so called therapeutic drug monitoring (TDM). Gene variants encoding proteins responsible for drug metabolism and transport, as well as drug actions, may explain some of the concentration/response variability. Our research group is involved in the optimization of drug therapy using TDM, pharmacogenetic analyses, biomarkers and other outcome measures. For drugs and toxic substances, we investigate the relationships between serum concentrations and effects/side effects.