In the history of humankind, the sea has always played a key role as a privileged medium for communication, commerce and contact among population centers. It constitutes an essential ecosystem, and an invaluable reservoir and source of food for all living beings. Therefore, its heath is a critical challenge for the survival of all humanity, particularly as one the most important environmental components targeted by global warming. Measuring and monitoring techniques are key tools for managing the marine environment and for supporting the Blue Economy. With this perspective, a series of annual international events, entitled Metrology for the Sea (MetroSea for short) was begun in 2017. Their increasing success inspired this book, which provides an anthology of tutorials dealing with a representative selection of topics of concern to a broad readership. The book covers two broad application areas, marine hydrography and meteorology, and then deals with instrumentation for measurement at sea. Typical metrological issues such as calibration and traceability, are considered, for both physical and chemical quantities. Key techniques, such as underwater acoustic investigation, remote sensing, measurement of waves and monitoring networks, are treated alongside marine geology and the monitoring of animal species. Economic and legal aspects of metrology for navigation are also discussed. Such an unparalleled wide vision of measurement for the sea will be of interest to a broad audience of scientists, engineers, economists, and their students.