This paper presents joint research between a Spanish team and an American one on the development and exploitation of a Spanish treebank. Such treebanks for other languages have proven valuable for the development of high-quality parsers and for a wide variety of language studies. However, when the project started, at the end of 1997, there was no syntactically annotated corpus for Spanish. This paper describes the design of such a treebank and its initial application to parser construction. 1. Constructing a Spanish treebank 1.1. Preliminary considerations This paper presents joint research between a Spanish team and an American one on the development and ex-ploitation of a Spanish treebank. Such treebanks for other languages have proven valuable for the development of high-quality parsers and for a wide variety of language studies. As there was no previous experience in building a syntactically annotated corpus for Spanish, the first effort consisted necessarily in writing a set of annotation guide-lines. The starting point was the existing documentation at that time, especially the Penn Treebank project (Marcus,