The research undertaken in this thesis is focused on the study of photomorphogenesis in tomato (Solanum lycopersicon). It includes a detailed physiological characterization of several previously described but largely understudied mutants that display light hyperresponsive phenotypes, as well as a reverse genetic study focused on the photomorphogenesis regulator DE-ETIOLATED 1 (DETl). A range of light- hyperresponsive mutants were selected in order to gain new insights into light signal transduction, including atroviolacea (atv), Anthocyanin fruit (Af), Punctate (Pn) and dark green (dg), which were studied alongside the better characterized high pigment (hp1 and hp2) mutants. Studies of these mutants are presented with an emphasis on anthocyanin pigment biosynthesis and distribution, chlorophyll accumulation, plastid development, and light regulated gene expression in different light conditions. Other work in the thesis has focused on DET1, mutations in which are responsible for the tomato hp2 mutation. To better understand the function and importance of DET1 in tomato, several transgenic lines were generated containing different DET1 gene constructs. Unexpectedly, phenotypes characteristic of DET1 suppression were observed, e.g., dwarfeess and higher carotenoid accumulation in fruits, and molecular analysis indicated in all cases that these phenotypes were a result of suppression of endogenous DET1 expression caused by post-transcriptional gene silencing (PTGS). In an attempt to harness the biotechnological potential of DET1 gene suppression in fruits and to avoid the collateral negative effects on plant growth, expression of the DET1 gene was then modulated only within the fruits using fruit-specific promoters. The results show that suppression of DET1 expression specifically in the firuits enhances carotenoid content but does not affect plant growth and development, and fruit-specific DET1 gene silencing was confirmed at the molecular level. These results provide a novel example of the use of tissue-specific gene silencing to improve the nutritional value of plant-derived products.