The major ion composition of the Earth's aquatic ecosystems plays a fundamental role in where species can occur. This is even true within freshwater ecosystems, where ecologists report biodiversity declines associated with increases in major ion concentrations. Our research group has been studying various aspects of osmoregulation in aquatic insects and other freshwater invertebrates. Inquiries include mechanisms of acclimation, interaction of salinity with other abiotic factors such as temperature and hypoxia, and the development of a lab based mayfly model for molecular, physiological and life history studies. In this talk I will give an synthesis/overview of the state of our understanding and discuss future research needs.