Root nodule symbiosis enables nitrogen‐fixing bacteria to convert atmospheric nitrogen into a form that is directly available for plant growth. Biological nitrogen fixation provides a built‐in supply of nitrogen fertiliser for many legume crops such as peas, beans and clover. Legumes (Fabales) interact with single‐celled Gram‐negative bacteria, collectively termed rhizobia, whereas members of three other Rosid orders (Fagales, Cucurbitales and Rosales) interact with Gram‐positive filamentous actinobacteria of the genus Frankia. In legumes, infection proceeds through intercellular and trans‐cellular channels termed infection threads. At the same time, cells in the root cortex are induced to divide and generate the tissues of the nodule. Nitrogen fixation normally takes place within specialised bacteroid cells enclosed within organelle‐like cytoplasmic compartments termed symbiosomes. The anatomy and physiology of root nodules both reflect a high degree of structural and metabolic integration between plant and microbial symbionts.