Sex ratios are among the most basic of demographic parameters and provide an indication of both the relative survival of females and males and the future breeding potential of a population. The observed sex ratio is a consequence of natural selection on the sexes and any anthropogenic effects of harvest. This allows wildlife managers to regulate animal harvests to maintain desired sex ratios. Conversely, by monitoring sex ratios, wildlife managers can assess how harvest regulations may influence the relative mortality rates of the male and female segments of a population. This chapter addresses two general approaches to estimating sex ratios. The most general approach uses direct field observations to estimate the population sex ratio. Finite sampling methods are used to estimate these ratios under a variety of scenarios. Choosing among these methods should begin with the structure and nature of the population under investigation. Species that form aggregates or herds need to be surveyed differently than populations of solitary individuals. Cluster sampling techniques are therefore presented. Unequal probabilities of detection are possible when making visual counts of animals. This consideration is important when probabilities of detecting animal aggregations of different sizes are expected to be a function of group size. The advantage of direct estimation methods for calculating sex ratios is that no assumption is required about the dynamics of the population being studied. Instead, adherence to the principles of probabilistic sampling is all that is needed to ensure reliable estimation of population sex ratios.