Deer antlers are deciduous bony structures, unique to Cervidae, which are cast and re-grown annually. Antlers are present in most deer species and typically only in males. The cycle of antler casting and regrowth is regulated by hormones influenced by photoperiod. Antler growth demands large amounts of calcium and phosphorus within a short period of time, which causes temporary partial demineralization of the skeleton. Antlers and horns were used by Darwin as examples of exaggerated structures arising from the process of sexual selection. In fact, antler size and sexual dimorphism in body size have tended to increase throughout the evolution of Cervidae. This chapter focuses on the evolutionary significance of antlers. Antler shape, the presence of pointed tips, and their use in conspecific fights for access to mates and defense against predators support their function as weapons, although some evidence also suggests a possible role in visual communication, probably signaling dominance to rivals and mates. Antlers influence individual fitness, but entail costs that may affect individual longevity. In large-sized species, antler growth is energetically costly and increases tooth wear, thus reducing expected lifespan. This cost–benefit balance may promote selection for plasticity. The effort that a male can invest every year in antler development (including size, bone density, symmetry) depends on many factors such as age, body condition, and the level of male–male competition in the social environment. Antlers have widely attracted the attention of humans as tools, ceremonial objects, hunt memories and trophies, indicators of environmental conditions, medicines, and models for the study of tissue regeneration, osteoporosis, and cancer treatment.