Arsenic (As) is a widely occurring environmental contaminant. To assess human exposures to As, public health officials and researchers often conduct biomonitoring. Samples of urine, hair, nails, or blood are collected from potentially exposed people and are analyzed for As compounds and their metabolites. When analyzing for As exposure, it is useful to distinguish between As species, since they differ in their origin and toxicity. Urine is the most frequently used biological medium for biomonitoring. Measuring the urinary concentration of As is useful in assessing recent exposure to As, and high-quality reference ranges are available for urinary As concentrations in the U.S. population. Biomonitoring for As in hair and nails has been used in many studies and is particularly useful in evaluating chronic exposures to As. Interpreting the health implications of As concentrations in biological samples is limited by the small number of studies that provide information on the correlation and dose-response relationship between biomonitoring test results and adverse health effects. This study discusses the advantages and limitations of biomonitoring for As in biological samples and provides illustrative case studies.