Tropical forests are experiencing drastic human land use changes such that currently half of the world's tropical forests are regenerating secondary forests (FAO, 2020). This has spurred immense interest in restoration efforts, such as ARF100, the Borneo Initiative, Initiative 20x20, and Forest Stewardship Council, to aid the natural regeneration of tropical forests (UNEP & CBD, 2018; UNIQUE, 2020; Vergara et al., 2015). Restoration ranges from passive to active strategies, such as natural regeneration to active species selection and inoculation with native soil. However, restoration efforts are often not effective (Lindenmayer, 2020). One of the reasons is the lack of connection with ecological theory and processes (eg, competition, dispersal, succession), particularly with belowground processes. Belowground processes are relatively understudied (Averill et al., 2022; Seidl & Turner, 2022; Werden et al., 2022), yet the interactions of plant roots, soil microbes, and soil properties drive critical belowground ecosystem services such as erosion control, water and nutrient cycling, and soil carbon sequestration. Attention to these critical interactions has the potential to substantially increase the success of restoration efforts. Here we highlight the most important belowground processes for active tropical forest restoration (Figure 1), when specific restoration practices should be considered, and suggest key research priorities. Belowground processes are often mediated by microorganisms, which create critical links between plants and soil (Figure 1b). These plant–microbial interactions can occur via established symbioses where microbes are physically connected to plant roots (eg, symbiotic nitrogen-fixing bacteria and mycorrhizal fungi) or via the free-living soil microbial community (eg, through root exudation and soil pathogens). Interactions between roots and soil microbes often drive reciprocal effects between plants and the soil, called plant–soil feedbacks (PSF), which can make the local soil either more (positive PSF) or less (negative PSF) favorable to particular plant species. These plant–microbe interactions are important for forest recovery, as highlighted by a global analysis showing plant biomass recovered 64% faster after inoculating plants with microbiomes from native ecosystems (Averill et al., 2022). Disentangling the multitude of plant–soil–microbe interactions and their impacts on the many belowground processes relevant to tropical forest restoration can be challenging. We structure our article around several key restoration objectives: soil stabilization and carbon storage, nutrient cycling recovery and forest growth, and forest diversity and multifunctionality. Here we define forest multifunctionality as: a forest ecosystem that can provide a mix of environmental,