In this thesis, I explore why biodiversity, as exemplified by diving beetle and mosquito faunas, differs between wetlands, and what environmental factors that affect and control diversity in different types of wetlands and landscapes. The study organisms, diving beetles (Dytiscidae) and mosquitoes (Culicidae), are abundant and species-rich groups in many types of wetlands. They also represent predator and prey. I have explored the diversity of diving beetles and mosquitoes along environmental gradients, along which primarily permanence, size, age, and shading, differ between wetlands. I have also studied wetlands in different types of landscapes (urban and agricultural), and in two different geographical regions (SE and SW Sweden). I have studied the colonization process of new wetlands by diving beetles, and changes in their faunas during the ecological succession of wetlands, and also migration and dispersal of diving beetles in different types of landscapes. Finally, I experimentally explored interactions between the predacious diving beetles and a potential prey - mosquito larvae. One important goal throughout the work has been to formulate advices for construction and management of wetlands, to obtain or preserve high biodiversity, both in individual wetlands as well as in whole landscapes. The results illustrate that many types of wetlands: permanent and temporary, small and large, situated in forests and in open environments, new and old ones, support high diversity of the studied organisms. Urban wetlands had generally fewer species than wetlands in agricultural landscapes, but wetlands in both types of landscapes supported unique species. The faunas differed considerably between wetland types, as many species have specific habitat requirements, but it was hard to classify faunas in specific wetland types as more "valuable" than others. Hence, to obtain a rich wetland insect fauna on a landscape level (which should be more important than to maximise diversity in individual ponds), it seems necessary to take into account more than individual wetlands or ponds. It might be vital to maintain different successional stages, and to construct new wetlands (or to rejuvenated later stages), since the early stages are short-lived, and support both rare and unique species not found in later stages. The interaction experiments, among dytiscids and mosquito larvae, showed that diving beetles (middle-sized species, 10-20 mm) significantly reduced levels of mosquito larvae. Single dytiscids could consume large numbers of such larvae. These predators are probably important in the natural control of mosquito larvae.