In sun-loving plants, detection of the proximity of nearby competitors triggers a set of responses known as the shade avoidance syndrome (SAS). These responses will help the plant to acclimate to the proximity of vegetation that might compromise light availability and limit plant development. Plants sense the presence of nearby competitor vegetation as changes in the light quality, i.e. a reduced red (R) to far-red light (FR) ratio (R:FR). Among the various responses to neighboring plants, one of the best studied and characterized is the promotion of the hypocotyl elongation in seedlings of the model plant Arabidopsis thaliana, that might help them to outcompete neighboring seedlings and reach better light conditions. In addition to this and other developmental changes, shade-perception affects photosynthetic capacity and reduces plant defense. In this chapter, we will review the main molecular aspects that control the regulation of hypocotyl elongation, including the role of the phytochrome photoreceptors, that sense the signal, and the best known genetic and hormonal components that participate downstream shade perception. Because of the obvious interest for translating this knowledge to agriculture, we will also explore what is known about the molecular interaction between shade perception, defense responses and the growth-defense trade-off observed in high planting density.