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Legumes are ecologically and economically important plants that contribute to nutrient cycling and agricultural sustainability, features tied to their intimate symbiosis with nitrogen-fixing rhizobia. However, rhizobia vary dramatically in quality, ranging from highly growth- promoting to nonbeneficial. Therefore, optimizing plant benefits from this symbiosis requires host mechanisms that select for beneficial rhizobia and limit losses to nonbeneficial strains. Here, we examine the considerable scientific progress made in decoding host control over rhizobia, empirically demonstrating both molecular and cellular mechanisms and their effects on symbiotic benefits. Pre-infection control requires plant production and detection of precise molecular signals to attract and select compatible rhizobia strains. Post-infection mechanisms leverage nodule- and cell-level compartmentalization of symbionts to enable host control over rhizobia development and proliferation in planta. These layers of host preferential allocation act as a series of sieves, each of which contributes to legume fitness by directing host resources to a narrowing subset of more-beneficial rhizobia.