Persistence of aquatic insect populations depends on successful recruitment of aquatic juveniles via oviposition by terrestrial adults. New recruits could arise from oviposition by adults that have dispersed from distant populations (immigration) and/or adults that emerge locally (self-recruitment). Freshwater ecology has traditionally focused on events that affect juvenile aquatic insects with scant attention to the links between terrestrial and aquatic life stages, despite their pivotal role in population dynamics. This talk highlights how oviposition behaviours can influence population size, and how this varies with interactions between species and with the physical environment. Studies at multiple spatial scales demonstrate associations between the abundance of oviposition sites and egg masses in rivers. This spatial variation can also influence densities of larvae, demonstrating potentially long-lasting effects on population size. Seasonal variations in oviposition site abundance may have little impact on populations in rivers with natural flow regimes; the effects of interannual variations are less clear. Oviposition site selection is species-specific and very selective behaviours suggest fitness benefits for adults or offspring, or both. For offspring, egg-hatching success can vary with environmental conditions and egg predation. Eggs of aquatic insects are typically spatially aggregated, but the spatial scale and aggregation strength depend on interactions among females (congregation vs avoidance). Such aggregation may affect density-dependent processes during larval life and hence influence population size. Despite recent advances, a more comprehensive understanding of oviposition is required to effectively integrate population and community ecology for aquatic insects.