Perfect crystals of biological macromolecules are needed to reveal structural information necessary for the understanding of their functions. Weightlessness conditions encountered during orbital space flights have been used for the past 20 years to grow better and larger crystals. The facilities and instruments used until now to grow crystals in space have primarily focused on growing crystals for post-flight analysis, and to a lesser extent on the understanding of phenomena associated with the crystallisation processes. The Protein Crystallisation Diagnostics Facility (PCDF), a new facility presently under development at ASTRIUM under a contract of the European Space Agency (ESA), will fly on the International Space Station in the 2004-2006 timeframe. This new instrument will allow for the observation and study of the crystallisation processes of biological macromolecules over long periods in microgravity using advanced diagnostics methods, including video microscopy, dynamic light scattering, and Mach-Zehnder interferometry. The European scientific community is preparing for the utilisation of this new instrument. This paper presents the actual status of the main design features of the PCDF and addresses the scientific preparation programme.