<p>Thermal stress reduces fertility and fecundity in animals at temperatures below lethal. Reproductive output is impaired across taxa under diverse heat-exposure regimes, with consequences for individual fitness, population persistence and ecosystem dynamics. This pattern holds across terrestrial and aquatic systems, with implications for conservation, livestock, aquaculture and human health. Yet these sublethal effects remain underrepresented in biodiversity forecasts. In this Review, we synthesize evidence for the biological mechanisms associated with thermally induced declines in fertility and fecundity, and assess how life history and exposure regime can shape thermal sensitivity. Fertility-based thermal limits can predict species distributions and extinction risk better than survival-based measures, albeit tested across a limited taxonomic range. Evolutionary responses to fertility loss under warming seem constrained but increased mutational variation, local adaptation and hybridization might increase fertility resilience. Key research priorities include broader taxonomic evaluation of both evolutionary potential and ecological outcomes under more sophisticated conditions, assessing how fertility is affected when different environmental stressors interact, and understanding how community and ecosystem dynamics will change if fertility-sensitive taxa either shift distributions or go extinct. Recognizing and addressing fertility-based vulnerability is essential for anticipating biodiversity change and designing more effective responses to climate impacts.</p>