Improved clarity of Neanderthal ways of life brought about by advancements in analysing the fossil and archaeological records, accompanied by increased willingness to accept complex Neanderthal cognition, makes it appropriate to begin to understand their sexual behavior. In this chapter, we briefly review current understandings about Neanderthals based on anatomy, genetics, and behavior evidenced from the archaeological record. We then integrate this with broad behavioral ecology and evolutionary sexual selection concepts to consider potential selection pressures on Neanderthals’ sexual and reproductive behaviors. Large adult brain size, rapid infant brain growth, and protracted offspring development, similar to Homo sapiens, were supported by adaptations in social organization, mating and parental effort. It is likely that male provisioning and investment in offspring strengthened reproductive pair bonds, improved infant survival, and impacted mate choice in both sexes. Systematic collaborative subsistence strategies were probably matched by a heavy reliance on kin and other trusted adults within the cooperative breeding group, reducing the energy burden on reproducing females, and enabling shorter lactation and reduced interbirth intervals. Neanderthals’ wide ecological tolerances and behavioral flexibility suggest that they also adjusted their sexual and reproductive behavior according to environmental circumstances. Small group size, local-to-regional social networks and potentially seasonal breeding enabled populations to adapt to fluctuating energy availability. During harsher climatic phases, limited access to mating opportunities may have favored social monogamy, with genetic isolation and inbreeding more likely. When conditions were milder (during interglacials, in warmer regions or seasons) with more plentiful resources, group sizes and social networks may have permitted polygyny. Finally we explore the behavioral implications of genetic evidence that Neanderthals interbred with other hominins including H. sapiens. This suggests that differences in physical appearance and social structures did not prevent copulation or raising hybrid infants, although sterility and lower fitness of the latter may have limited the spread of genes between species.