Estimation of key population sizes is essential for advocacy, program planning, and monitoring of HIV epidemics in these populations. A review of recent publications on population size estimation among key populations including MSM, people who inject drugs, and male and female sex workers was conducted to identify and assess current practices at the global level.Studies have used multiple methods including capture-recapture, service multiplier, and unique object multiplier. Other studies apply census and enumeration, often before implementation of a behavioral survey. Network scale-up is used infrequently. Newer methods or variations of existing size estimation methods have emerged that are applied solely within surveys.A range of size estimation methods is available. All methods rely on theoretical assumptions that are difficult to meet in practice, are logistically difficult to conduct, or have yet to be fully validated. Accurate and valid key population size estimates remain as necessary as they are challenging to undertake; the concurrent use of multiple methods may be justified to facilitate the triangulation and interpretation of the resulting estimates. Formative assessment can help inform the appropriateness and feasibility of different size estimation methods.