Purpose – This article aims to review the different devices that are available for the in situ monitoring of analytes found in the marine environment. Design/methodology/approach – Following a short introduction to the topic, this paper discusses physical- and chemical-based sensors, automatic analysers (flow injection, spectroscopic and spectrometric), electrochemical devices and biosensors. Findings – A wide range of in situ monitoring systems (and associated deployment apparatus) for measuring concentrations of various analytes (e.g. nutrients, organic chemicals and metallic elements) have been developed in recent decades. Many of these systems are still at the laboratory or prototype stage and are yet to be fully developed into commercially available products. The harsh conditions often found in the marine environment can further limit the utility and application of these sensors. Further development work is needed; however, the need now is for field deployments, validation and inter-calibration between sensors and other analytical measurement techniques. Originality/value – This paper provides up-to-date information on in situ technologies that are available, either at the laboratory and prototype stages or commercially, and are suitable for deployment in the marine environment. Applications of these sensing systems are discussed together with their associated advantages and disadvantages over other existing water monitoring methods.