Part I of this series defines biomarkers and discusses their research utility and potential application in clinical practice. To provide a frame of reference, biomarkers commonly used in general medicine are reviewed, with a focus on low density lipoprotein as a biomarker for risk of developing atherosclerosis. The use of biomarkers in research on psychiatric illnesses is then reviewed. While biomarkers currently have only a limited role in psychiatric care, their use has improved our ability to assess potential efficacy and safety of investigational new drugs. For example, positron emission tomography can be used to measure dopamine D2 receptor occupancy (relevant for antipsychotic effects) or serotonin transporter occupancy (relevant for antidepressant effects), to establish whether an agent reaches and to what extent it affects a specific target in the brain. In the future, biomarkers are likely to become an integral component of psychiatric treatment, providing information concerning a patient's odds of developing an illness, diagnosis, severity of illness, and level of response to therapeutic interventions. The second part of this series will discuss research on derivatives of the inflammatory biomarker thromboxane and depression.