Bioavailability estimates the actual internal uptake or absorption of contaminants that enter the body (internal dose) and helps in providing a more accurate estimation of the human risks than the usage of total concentration. This is important for exposure assessment for children in relation to their hand-to-mouth activities. For example significant reductions of the bioavailability of long-term contaminated soils have been demonstrated using various animal models. The measurement for bioavailability involves various uncertainties for organic contaminants. It is crucial to determine the parameters that influence the results of bioavailability. This chapter provides a summary of the current state of knowledge for the determination of bioavailability for a range of organic contaminants. The information provided will be useful in facilitating further research efforts for the investigation of bioavailability of contaminants in conducting exposure assessments.