During the past few years, the tomato carotenoid, lycopene, has become increasingly recognized as an important dietary phytochemical with unique value in human health. The accumulating epidemiological evidence supporting the association between diets rich in the tomato products, therefore lycopene, and certain health benefits are very compelling, leading to an increasing interest the stability and bioavailability of this phytochemical in the diet. Meanwhile, the consumption of tomatoes and related products in the United States are on the rise. Furthermore, the ratio of lycopene in the all- trans geometrical forms versus those in the cis configuration from fresh tomatoes has been observed to be significantly different than those from human blood and tissues. This difference, specifically, has prompted considerable attention to the formation and potential biological significance of lycopene isomers, especially in terms of possible unique or altered physiological roles associated with a particular geometrical configuration. Some investigators have suggested that lycopene undergoes isomerization reactions during thermal processing of tomato products and the level of newly formed species in the cis configurations are reflected in the blood and tissues following absorption and distribution. A definite account of lycopene isomerization processes and end points requires extensive experimental data which are not currently available. The objectives of this work are to monitor the stability of lycopene and other tomato carotenoids towards isomerization during typical food processing using various tomato varieties and secondly, to determine if isomerization reactions take place during the digestive process. First, the thermal stability during food processing of lycopene in the all-trans geometrical configuration was evaluated using industrial processing facilities and conditions. The results indicated that, lycopene, in contrast to ß-carotene, was stable against isomerization reactions during thermal processing and following an 18-month storage period, independent of the production variables such as product type, container type, moisture content, tomato variety and severity of heat treatments. Second, when tomato mutant varieties with distinct differences in the distribution of chromoplastid carotenoid pigments, including ß-carotene, d-carotene, y-carotene, lutein and prolycopene-a poly-cis lycopene isomer, were heat-treated in either distilled water or a water/oil mixture, lycopene, d-carotene and y-carotene were also found to remain stable in the all-trans configurations. ß-carotene and lutein, on the other hand, readily isomerized to the cis geometrical forms as a result of both thermal treatments, suggesting either an isomerization mechanism involving either localized specificity within tomato cells or structural selectivity among tomato carotenoids. Third, no isomerization of tomato carotenoids including lycopene occurred either during the gastric or intestinal phases of digestion as monitored by an in vitro digestion protocol. Additionally, high levels of lycopene cis-isomers were observed in Caco-2 human colon cells, which have been incubated with micellar lycopene derived from the in vitro digestion protocol. Chromatographic analysis of biological samples such as human blood chylomicron fractions from individuals consuming lycopene-containing products, human serum and prostate tissues, likewise, yielded high cis to trans isomeric ratios. These findings help to confirm that the high level of various lycopene cis isomers found in biological fluids and tissue are not formed during food processing, food storage or during digestion, and therefore most likely arise after absorption. Ultimately, these findings not only provide a better understanding on the impact of food processing and operational variables on lycopene’s stability in vitro but they also contribute toward the overall effort to assess nutritional and physiological implications of this phytochemical in the diet. The implications of this knowledge will have an impact on basic lycopene research as well as in the design of lycopene intervention trials in humans.