There is growing need for accurate information regarding the bioavailability of carotenoids, both with respect to carotenoids per se and to the vitamin A value of provitamin A carotenoids in foods or supplement preparations. Little quantitative information is currently available, owing primarily to the lack of adequate methods to assess carotenoid bioavailability. Methods applied to xenobiotic drugs are in most cases not useful for carotenoids, many of which circulate in appreciable quantities in human plasma. Reported ranges of carotenoid bioavailability (% dose absorbed) range from 1-99, and variability is generally high both within and between treatments. With the current methods, relative bioavailability is more readily assessed than absolute bioavailability. The most commonly applied methods include measuring the increase in plasma carotenoid concentration following chronic intervention, and use of postprandial chylomicron (PPC) carotenoid or retinyl ester response following a single dose of carotenoid. The advantages and limitations of these approaches, together with examples of each, are discussed. A new PPC approach utilizing extrinsic-stable-isotope-labelled vitamin A (2H4-labelled retinyl acetate) is under development in our laboratory, and examples of its application are presented. The currently available data suggest that oil solutions of carotenoids are more bioavailable than those from food matrices, and heating can improve the bioavailability of carotenoids from some food products. Increased availability of labelled carotenoids and retinoids should aid the development of reliable methods of carotenoid bioavailability assessment. Such data are needed for dietary recommendations, supplement formulation, and design of intervention strategies involving carotenoids.