Study on the Effective Mitigation Measures of Coastal Erosion due to Coastal Developments We investigated the effective methodology in the Environmental Impact Assessment (EIA) of coastal erosion due to a variety of coastal developments. The study focuses mainly on the sand and pebble beaches which are very active to coastal processes and attract social attention. Firstly, the coastal erosion types are classified to guide effective EIA in the coastal development. Then EIA processes for each type are provided such as field survey, impact assessment. mitigation options, and monitoring. For the developments such as port and harbour which are known to cause coastal erosion, specific field survey, numerical & hydraulic model experiment, impact assessment, and monitoring are provided. Basically, the environmental objective of the coastal erosion by a certain development project should be erosion-free. The objective can be firstly achieved in terms of primary mitigation options such location, scale, arrangement, erosion-friendly method of construction, and so on, In addition the secondary option to maintain, repair, and restore can be used such as soft option such as beach nourishment and hard on such as groin. Because of the uncertainty on the scientific knowledge of coastal erosion, the monitoring after construction should be importantly treated. Through monitoring, we have to compare the EIA assessment, to manage the eroded area, to identify the effect of mitigation option, and to treat with unexpected situation. In addition to monitoring, the scientific uncertainty in the coastal erosion field ask us to deal the coastal erosion field with evolving one, which includes all EIA processes of coastal erosion such as field survey, impact assessment, and monitoring as well as mitigation measures. The scientific uncertainty requires new information and method for coastal erosion. The empirical information can be preferred to numerical one under circumstances.