Ordovician trilobites are reviewed based on a new species-level relational database. The stratigraphical ranges of all 56 families with occurrences in the Ordovician are documented and the content, phylogenetic status, diversity and Ordovician distribution by major palaeocontinent/terrane are discussed. Aspects of higher classification are also dealt with. Global sampling is heavily biased towards a small number of highly sampled areas. Much of the world has a very limited record of formally named trilobite species. Even within heavily sampled units, sampling is patchy by environment and time. Genus endemism was at a peak in Laurentia, Baltic, and Avalonia in the Floian and declined more-or-less steadily through the remainder of the Ordovician.