The term “mean sea level”; is discussed, and a number of factors known to cause changes in mean sea level are reviewed. These factors include changes in water density ("steric”; sea level, or “dynamic height") and atmospheric pressure. In the open ocean, these factors, in addition to tides, are the main sources of variation recognized by oceanographers. Other sources of variation affect mean sea level as deduced from coastal tide gauges. These include continental shelf waves, local wind effects (wind and wave “set‐up"), and perhaps radiation stress due to planetary waves. Problems in the measurement of mean sea level are discussed briefly.