Although unification based grammars have proved useful in developing sentence grammars, little attention has been paid to the problem of extending the unification based approach to discourse grammar. This talk surveys a number of implemented unification based NLP systems and discusses their respective adequacy for the development of a unification based discourse grammar intended to deal with such phenomena as intra and intersentential nominal anaphora, temporal anaphora and discourse parallelism. The survey is both theoretical (i.e. based on the documentation) and practical (i.e. based on practical experience) and concentrates on defining and assessing the following system properties: expressivity, usability, state of development, efficiency and extendability.General points addressed: expressivity, theory commitment (can we express the relevant generalisations?) and practical usability of the available systems (i.e. practical efficiency) in relation to the development of a unification based discourse grammar.