Standard techniques used in multilingual terminology management fail to describe legal terminologies as they are bound to different legal systems and terms do not share a common meaning. In the LexALP project, we use a technique defined for general lexical databases to achieve cross language interoperability between lan- guages of the Alpine Convention. In this paper we present the methodology and tools developed for the collection, de- scription and harmonisation of the legal terminology of spatial planning and sus- tainable development in the four lan- guages of the countries of the Alpine Space.