Echimyid rodents are widely distributed and abundant throughout most of the Neotropics. I review the available information on spacing patterns and mating systems and develop a general framework for understanding variation among populations and species. In most studied species males have larger home ranges than do females, and overlap with either sex can range from 0% to nearly 100%. Males typically display more overlap with both other males and females than do females. Although lack of overlap has been interpreted as territoriality, most such cases have been from populations at low densities. Individuals of those same species show considerable overlap at higher densities. I suggest that individuals typically are not territorial but instead merely space themselves out when vacant space is abundant. Individuals are then facultatively monogamous because multiple mates are not available. As densities increase male home ranges overlap more simply because their home ranges are larger than those of females. Males then can acquire multiple mates, and polygyny becomes evident. At the highest densities multiple mates are available for both males and females, and promiscuity results. The mating system within a given population of many echimyids therefore depends upon density, which in turn is determined primarily by resource availability.