Japan has a long stretch of coastal dykes along its shoreline to protect against many types of coastal disasters. The 2011 Tohoku Tsunami caused serious damage to these coastal dykes along the long coastaline. One of the main reasons of the coastal dyke failure was overflowing tsunamis. Thus, after the 2011 Tohoku Tsunami, many studies on coastal dyke failure due to overflowing tsunamis have been conducted. The present study aimed to analyze the basic hydraulic characteristics of overflowing tsunamis on coastal dykes. First, the characteristics of overflowing tsunamis on coastal dykes observed during the 2011 tsunami were described using field data. The field data showed that there were different types of overflowing tsunamis during the 2011 tsunami and these types could be related to coastal dyke failure. Then, flow geometry were discussed based on the classification presented by Hom-ma (1940). Finally, the relationships between the hydraulic characteristics of overflowing tsunamis around coastal dykes (velocity field and pressure profile) and coastal dyke failure were discussed in detail by means of both laboratory investigations and numerical experiments.