Oceans, lakes, rivers, and groundwater are complex, dynamic environments in which physical, chemical, and biological processes occur over varying temporal and spatial scales (e.g., eddies, nutrient fluxes, patchiness of organisms, benthic processes, and pollution). In addition, deep, remote, or hostile systems such as hydrothermal vents and polar regions traditionally are poorly sampled, but are important to understanding global biogeochemical and hydrological cycles. In the coming decades, moored, cabled, and autonomous observatories will be used to investigate a spectrum of basic processes in aquatic environments. In anticipation of the need to develop or re-engineer sensors to measure physical, chemical, biological, and geological processes in situ, a one-day workshop and special session on sensor technology was held during the June 2000 meeting of the American Society of Limnology and Oceanography (ASLO) in Copenhagen, Denmark. The goal of the workshop was to exchange ideas on new experimental approaches and methodology, to define strategic themes, and to formulate specific recommendations related to sensor development. The 25 participants from North America and Europe represented academic and industry sensor developers and users, as well as a broad spectrum of scientific interests. Reported here are the recommendations resulting from that meeting in hope that they will be useful as a catalyst for further development of sensor systems.