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More formally, epidemiology is a health science that studies the distribution of diseases or adverse health effects in populations and the factors that modify, or that are suspected to be capable of modifying, the distribution of a disease. The purpose of studying disease distribution in a population is basically for one or both of the following reasons: (1) To determine if there is a health crisis or risk with the disease at issue; or (2) to use the disease distribution observed under some specific conditions as a measure of the association’s strength between the disease and a suspected hazard or factor. The selection of certain hazard or factor for epidemiologic investigation is rarely a random act. It is usually based upon some physical or biological, but even more typically, some toxicological understanding or data. That is, even though the assessment of the association process per se is statistical in nature, the formulation of this type of association is often based upon some biological or toxicological speculation. It might have been that by chance alone chimney sweeps were seen to be associated with higher incidence of cancer of the scrotum. Yet it was due to our acceptance of the underlying biological process that more epidemiologic investigations were conducted to confirm this link. The study of the distribution of, or the association with, the factor in a test population is an area involving some epidemiologic form of (human) exposure assessment. The basic tools used for this type of assessment studies include various forms of survey methods, biological monitoring, and some types of actual environmental or external personal measurements.