Insects are crucial for the functioning of ecosystems and might be facing declines globally, although data are biased away from the tropics where insect diversity and abundance are highest. In this Review, we assess the current status of insect populations in the tropics and discuss the prevailing threats to tropical insect biodiversity. Burgeoning human populations, increasing urbanization and land-use changes are leading to habitat loss and fragmentation, as well as increased pollution, including both light and pesticides. Insects on tropical islands are particularly sensitive to invasive species, which have already led to the extinction of multiple unique endemic species. Climate change further threatens insect populations across the tropics and might be disrupting crucial weather cycles such as El Niño and La Niña, which are important drivers of phenology and synchrony at these latitudes. Tropical insect declines might alter fundamental ecosystem processes such as nutrient cycling, carbon sequestration and herbivory. Disruption of food webs could lead to increased outbreaks of pests and of insect-vectored diseases in humans and livestock, affecting human health and reducing food security. Methodological advances — including artificial intelligence and computer vision, remote sensing and meta-barcoding — are facilitating taxonomy, speeding up identification of diverse samples and improving the monitoring of tropical insect biodiversity to guide future conservation efforts.