As a proof of concept, the qualitative and quantitative engineering of carotenoid formation has been achieved in crop plants. Successful reports in tomato, potato, rice, and canola all describe the enhancement of carotenoid with nutritional value, while in model systems such as tobacco and Arabidopsis the engineering of carotenoid to confer abiotic stress has been described. For all the successful applications there have been many examples of unintended/unpredicted phenotypes and results. Typically this has resided from our lack of understanding of carotenoid formation and its regulation. In the present article, we will review advances in carotenoid formation and its regulation to illustrate how metabolic engineering experiments have shed light on regulatory mechanisms.