The EU project CLASH (EVK3-CT-2001-00058) (Jan. 2002 - Dec. 2004), stands for 'Crest Level Assessment of coastal Structures by full scale monitoring, neural network prediction and Hazard analysis on permissible wave overtopping' (www.clash-eu.org). One of the objectives of CLASH is to produce a generally applicable prediction method for wave overtopping at coastal structures based on permissible wave overtopping and hazard analysis. The set up of a homogeneous database on wave overtopping is one of the main tasks within the framework of CLASH and is the main subject of this paper. On the one hand, the database gives a detailed inventory of overtopping tests which have been performed and on the other hand, the database serves as the starting point for a neural network prediction method. At this moment (Dec. 2003), the overtopping database consists of more than 6500 overtopping tests. This paper gives a complete overview of the contents of the database and discusses the methodology.