The Greek Taxon Information System is an initiative of the LifeWatchGreece Research Infrastructure (ESFRI) that is resuming efforts to compile a complete checklist of all species reported from the Greek territory. Such an effort is necessary as a requirement for all signatories of the Convention on Biological Diversity (Greece is a signatory since 1994). Over an estimation published in 2004 according to which 50,000 species are present in Greece, belonging to most kingdoms except bacteria and viruses, a list of 35,000 valid species (and subspecies) has been assembled from previous national and European initiatives and specialized databases on various groups. A new database will be progressively set up in the LifeWatchGreece Infrastructure within the near future. Before the dissemination of this dataset, it is important that the checklists will be validated by specialists for each taxonomic group. The first step already accomplished was to build and publish Preliminary Checklists for some taxonomic groups of marine fauna, which have been validated by specialists on the basis of their expertise and secondary literature. The publication of these Preliminary Checklists is expected to increase the visibility and usability of the database in the future not only to the scientific community but also to the broader domain of biodiversity management, especially in cases where no such checklists have been published yet. The guidelines used to test the first taxonomic groups are presented in this paper.