Since the 1970's, when the first in situ measurements of oxygen in the oceans were reported, the development of electrochemical sensors for in situ measurements in aquatic systems has significantly intensified. A synthesis of the progress made in limnology and oceanography to measure chemical species in situ is presented. From amperometric and potentiometric sensors that can measure a single analyte to voltammetric sensors that can measure several species during the same scan, a variety of electrodes have been used in situ to better understand the nature of the biogeochemical processes occurring in aquatic systems. The advantages and disadvantages of each technique, the technical improvements over the years, and some recommendations are presented together with representative data reported in the literature over the last two decades.