The perception of ambient light signals that produce a relevant response to ensure exposure to appropriate levels of light energy is vital for plants. In response to this, intricate molecular mechanisms to mediate light signaling have evolved in plants. Among the responses induced by light, seedling extension is a determining event for plant survival in darkness, especially in the initial stage of plant growth. Here we review previous studies and recent progress towards an understanding of light signaling that regulates seedling elongation. We focus on the three regions of the sunlight spectrum that primarily control seedling elongation, namely red/far-red light, blue/UV-A light and UV-B light, and summarize the four signaling pathways that correspond to the three effective spectra.