Light affects both the development and the metabolism of plants. In addition to the role of light in providing energy for photosynthesis, light signals cause profound changes in the morphology of the developing young seedling, including cotyledon expansion, leaf development, inhibition of stem growth, and production of chlorophyll in the photosynthetically competent chloroplast. The light-dependent development of plants (photomorphogenesis) is a complex process resulting from the combined action of several photoreceptors. This review summarizes what is known of the red- and blue-light photoreceptors that regulate dicotyledonous seedling development and the complexity of the downstream responses. Special emphasis is placed on the recent progress made toward genetic and biochemical dissection of the signal transduction pathways.