Tsunami is a Japanese word that is made of two characters tsu and nami. The character tsu means harbor while the character nami means wave.Therefore the original word tsunami describes large wave oscillations inside a harbor during a tsunami event. In the past tsunami is often referred to astidal wave which is a misnomer. Tides featuring the rising and falling of water level in the ocean in a daily monthly and yearly cycle are caused bygravitational influences of the moon sun and planets. Tsunamis are not generated by this kind of gravitational forces and are unrelated to the tidesalthough the tidal level does influence a tsunami striking a coastal area. The phenomenon we call a tsunami is a series of water waves of extremely longwavelength and long period generated in an ocean by a geophysical disturbance that displaces the water within a short period of time. Waves are formedas the displaced water mass which acts under the influence of gravity attempts to regain its equilibrium. Tsunamis are primarily associated withsubmarine earthquakes in oceanic and coastal regions. Tsunamis are characterised as shallow water waves or long waves which are different from windgenerated waves the waves many of us have observed on a beach. The initial amplitude of a tsunami in the vicinity of a source region is usually quite smalltypically only a meter or less in comparison with the wavelength. In general as the tsunami propagates into the open ocean the amplitude of tsunami willdecrease for the wave energy is spread over a much larger area. In the open ocean it is very difficult to detect a tsunami from aboard a ship because thewater level will rise only slightly over a period of ten minutes to hours. Since the rate at which a wave loses its energy is inversely proportional to itswavelength a tsunami will lose little energy as it propagates. Hence in the open ocean a tsunami will travel at high speeds and over great transoceanicdistances with little energy loss. As a tsunami propagates into shallower waters near the coast it undergoes a rapid transformation. Because the energy lossremains insignificant the total energy flux of the tsunami which is proportional to the product of the square of the wave amplitude and the speed of thetsunami remains constant. Therefore the speed of the tsunami decreases as it enters shallower water and the height of the tsunami grows. Studies ontsunami sedimentology have revealed that tsunamis induce various types of sedimentation in marine lacustrine and onshore environments. Tsunamideposits are sedimentological evidence of tsunami events. This contribution describes the nature of tsunamis and of tsunami sedimentation as anintroduction to this volume. Hydrodynamic aspects are introduced to explain the propagation of tsunamis. The diversity of tsunami deposits is illustratedon the basis of literature data. Onshore tsunami sedimentation is discussed in particular. Tsunami sedimentation appears to depend on the hydrodynamicand hydraulic character of the tsunami. The distribution pattern grain size variation and many other sedimentological structures reflect the characters ofthe tsunami such as the height current velocity and period. Therefore tsunami sedimentation should be interpreted based on careful consideration of thecharacteristics of tsunamis. This may result in a reliable reconstruction of ancient tsunami events.