Sea water level records provide observations of the height of the sea surface relative to a local height reference, as measured by a water level recorder (tide gauge). Tide gauge measurements represent relative sea level change, reflecting the combined effects of changes in the absolute sea surface height and vertical land motion at the gauge location. To derive absolute sea level change, tide gauge records must therefore be corrected for vertical land motion (see product ESD), which is commonly determined from continuous or repeated GNSS observations. Long-term averaging of tide gauge records reduces the influence of short-period temporal variability, while time series analysis provides valuable information on long-term sea level change, decadal climate variability, tides, storm surges, tsunami signals, meltwater input, and other coastal processes. Tide gauge observations are also indispensable for calibrating and validating satellite altimetry, assessing the performance of ocean circulation models, and detecting instrumental biases or long-term drifts in satellite-derived sea surface heights. In tectonically active regions, such as island arcs, historical tide gauge records provide unique evidence of long-term vertical crustal deformation and, in some cases, coseismic uplift occurring before the advent of space geodetic techniques. They therefore constitute an important source of information for studying crustal dynamics and are indispensable for reconstructing long-term sea level change. The following geodetic product is key to realising, monitoring and applying this Essential Geodetic Variable: