This paper presents a gratmnar and style checker demonstrator for Spanish and Greek native writers developed within the project GramCheck. Besides a brief grmnmar error typology for Spanish, a linguistically motivated approach to detection and diagnosis is presented, based on the generalized use of PROLOG extensions to highly typed unification-based grammars. The demonstrator, currently including flfll coverage for agreement errors and certain head-argmnent relation issues, also provides correction by means of an analysis-transfer-synthesis cycle. Finally, fltture extensions to the (:urrent system are discussed.