Tides in the ocean are generated by the gravitational forces of the moon and the sun with the earth, and the tide-generative force is the resultant of the gravitational force of the moon or sun and the centrifugal force due to the rotation of the earth. Tides are periodic and their variations occur in semi-diurnal, diurnal, fortnightly, monthly, annual and longer time scales. Observed tides can be considered to be the resultant of many harmonics, each considered as a tidal constituent. Tidal constituents, having definite periodicities, are defined to account for the complex motion of the moon around the earth or the earth’s motion around the sun. The combined tide-producing forces of the moon and sun result in a 14.75-day fortnightly spring-neap tidal cycle, with the largest tides occurring during spring tide and vice versa during neap tide. In this chapter, various types of instruments for measuring sea level including mechanical tide gauges and modern electronic tide gauges and analysis of data are briefly described. Tides generated in the open ocean have small amplitudes, but when they approach towards the coast, they amplify. In the north Indian Ocean, large tides are observed in the Gulf of Khambhat and in the Gulf of Kutch, along the west coast of India and in the estuaries in the Sundarbans region in the head of the Bay of Bengal. Large semi-diurnal tides occur along the northern part of the west coast of India and in the head of the Bay of Bengal due to amplification occurring on wide continental shelves. Phase of the tide during the occurrence of an extreme event (a storm surge or a tsunami) is critical, as the events occurring during high tide tend to have large total water levels at the coast and could cause more destruction than for the same events if they occurred during low tide.