The assessment of biological parameters in relation to psychosocial, emotional, and behavioral factors is a cornerstone of behavioral medicine. This chapter provides an introduction to the use of biological variables, focusing on their measurement in three principle settings: epidemiological and population studies, psychophysiological stress testing in the laboratory or clinic, and naturalistic and ambulatory monitoring in everyday life. Each paradigm has strengths and limitations and serves particular purposes in behavioral medicine. Integration of knowledge obtained from these different strategies is needed in order to generate a comprehensive understanding of the relationship between biological parameters and the social and psychological environment in health and disease.