Spatial conservation planning has emerged as a powerful tool that leverages spatial ecology to guide area-based conservation. Systematic conservation planning techniques have advanced rapidly over the past two decades to provide objective, quantitative tools for identifying areas that are most important to reaching conservation targets and those that are most irreplaceable. Here, we provide an overview of modern systematic conservation planning and provide an example with protected area prioritization for mammals in Eswatini. We first describe the history of conservation planning that emerged from the Single Large versus Several Small debate. This debate and related issues, such as the use of species–area relationships for conservation planning, were limited in their scope. Modern systematic conservation planning emerged to address some of these limitations and was largely based on the idea of complementarity, where a focus is on how different sites (e.g., protected areas) provide complementary species assemblages to reach over-arching conservation targets. These modern methods also built in the ideas of representation, irreplaceability, adequacy, and vulnerability formally into planning techniques. Several objectives have been recognized, such as the maximum coverage and minimum area (or cost) problems. Our example illustrates how to plan for protected areas, acknowledging different factors in the prioritization process, such as connectivity and edge effects. We end by discussing some increasingly common extensions to planning, such as planning for multiple objectives and climate change.