Tides are the rising of Earth's ocean surface caused by the tidal forces of the Moon and the Sun acting on the oceans. Tides cause changes in the depth of the marine and estuarine water bodies and produce oscillating currents known as tidal streams, making prediction of tides important for coastal navigation. The strip of seashore that is submerged at high tide and exposed at low tide, the intertidal zone, is an important ecological product of ocean tides. The changing tide produced at a given location is the result of the changing positions of the Moon and Sun relative to the Earth coupled with the effects of Earth rotation and the bathymetry of oceans, seas and estuaries. Besides the ocean, tidal phenomena can occur in other systems whenever a gravitational field that varies in time and space is present. The daily sea-level fluctuations, as they actually occur, especially in shallow seas and near coasts, are not solely caused by the astronomical tidal forcing, but may also be strongly affected by meteorology (wind and barometric pressure), for instance resulting in storm surges.

The gravitational pull of the Moon and the Sun both affect the Earths tides. When Sun, Moon, and Earth are in alignment (at the time of the new or full moon), the solar tide has an additive effect on the lunar tide, creating extra-high high tides, and very low, low tides both commonly called spring tides. One week later, when the sun and moon are at right angles to each other, the solar tide partially cancels out the lunar tide and produces moderate tides known as neap tides. During each lunar month, two sets of spring and two sets of neap tides occur.