Phytochemicals are increasingly accepted as health promoting, maintaining, and repairing agents in cells, tissues, or the whole human body. Phytochemicals are compounds obtained from plants that exert particular health effects; generally, they are not necessarily basic nutrients (minerals, vitamins, carbohydrates, proteins or lipids), medicines or toxins. The phytochemicals that are frequently associated with human health are phenolics, carotenoids, organic acids, and several miscellaneous bioactive compounds such as saponin and sterols. The contributions of phytochemicals in public health cover various issues world-widely and thus it is seen by researchers, industries, general society, and policy makers as a new tool to manage public health. Ironically, the roles of phytochemicals in health are poorly understood, which warrant the needs for validation as well as scientific database on safety issues and mechanisms of the functions. Even though various genetic-base studies propose mechanisms and health interventions of phytochemicals (Noe et al., 2004), many findings are inconclusive. Hence, the emerging health potentials of phytochemicals are inconclusive; and internationally it has been the reason for new policies/regulations in food trading. This is partly due to limited understanding on phytochemical bioavailability by which the health benefits depend on. Moreover, transport mechanisms for phytochemicals delivery into the target sites, phytochemical metabolisms by the human body, and biomarkers exerting the health benefits are also poorly understood. These complexities call for a new framework on how and to what extent dietary phytochemicals should be recommended in order to reach biologically-safe active dosages. In the human body, bioavailability is defined as substances obtained from ingested materials that reach circulatory system for further delivery into designated tissues so that the beneficial compounds are biologically available for exerting healthy functions. The normal routes of dietary phytochemicals thus include ingestion, digestions, and transport across gastrointestinal epithelium prior to circulatory vessels. The epithelium in the gastrointestinal tract is a polarized enterocyte cell having two different sides facing luminal hollow (Apical side) and blood capillaries (Basolateral side) where each side is equipped with different transport facilities and barriers. The epithelial cells are critical for bioavailability of target compounds either as entrance gates or as metabolizing machines which release different compounds from the parent molecules. These make further complexing bioavailability routes because the metabolisms and transport processes are also involved in the orchestrated physiological regulations maintaining homeostasis states of the human body. However, bioavailability of phytochemicals by which the health benefits depend on are not well understood; consequently, it is difficult to be measured. The difficulties in studies of bioavailability are mainly due to the complexities involved in the biological system, i.e. (a) variation in food materials and the human subjects or surrogate models which are not always representative; (b) complex interactions amongst huge chemicals/food components during postharvest, storage, processing, digestion, and absorption that may alter health benefits; and (c) mechanism pathways. In this paper, fundamental aspects of phytochemical bioavailability are reviewed.