Most of the world’s coastal areas have been shaped and transformed by tsunamis from prehistorical times with origins varying from cosmically related events to tectonic plate dynamics and atmospheric disturbances. The most devastating tsunamis involve large meteorite strikes, followed by massive submarine landslides and volcanic eruptions. However other comparatively devastating tsunamis have originated from earthquakes, surface landslides and seafloor displacement. Since 1960, four major tsunami events can be considered to be the most relevant as a result of their size and associated destruction: two events off the coast of Chile, one in the Pacific and Indian Oceans that reached many countries along its path, and the March 2011 Great East Japan Earthquake and Tsunami. The latter badly damaged infrastructure, the social milieu, local communities and cities, industries and the environment of Japan’s northeastern coast, prompting the government to engage in an intense 10-year restoration and reconstruction process. The environment vastly suffered the impact, which was reflected in important biodiversity alterations as well as in the presence, abundance and distribution of various coastal ecosystems and biological species. Apart from the ongoing natural restoration process, Japan’s government has also decided to support and enhance the process that has developed and has enacted important statutes and statutory frameworks for this purpose, including the 2012–2020 National Biodiversity Strategy, which turns on certain fundamental components, such as the valuation of ecosystems services, and implementation of Environmental Impact Assessments and Environmental Strategic Assessments, among others. Controversial issues, including construction of sea fences (such as seawalls or coastal dikes), the potential damage to the coastal environment, effectiveness and costs of certain measures, among others, resulted from the established reconstruction policies and differences in perceptions between the government and local inhabitants throughout the reconstruction process. As result of intense interaction between the stakeholders and the government, many of the initial decisions regarding the characteristics of these structures have been revised, although their impact on the environment will certainly remain large and often unpredictable. Although the present chapter is not intended to be a comprehensive analysis nor a compilation of information, it is intended to provide a brief overview regarding tsunamis, in terms of their origin and types, while giving some relevant examples of the most devastating tsunamis in historical terms and since 1960 to date as a result of their environmental impacts. A deeper analysis is made of the March 2011 event in Japan in which some examples are offered regarding the decimation and or alteration of the biodiversity and ecosystems in some of the affected coastlines. Moreover, this chapter highlights some efforts that Japan’s government has undertaken in the 5Â years since March 2011 by developing and enacting relevant legal frameworks and related aspects. Finally, this chapter also addresses the contents of the discussion regarding construction of sea fences along the coastline in the Tohoku region (northeastern coast of Japan).