I am building an Access database containing research data from various projects. The main table in the database is the Species table, which has SpeciesID as the primary key. Each species has many possible attributes starting with taxonomy eg. Class, Order, Family, Genus, SpeciesName, and continuing through many other attributes, which may or may not be recorded for each species. For each attribute, for each species, I need to record which project the data has come from (I have a Project table with ProjectID as the primary key), and the reference used (I have a Reference table with RefID as the primary key). I have tried having a SourceID (links to project) and RefID (links to Reference) column after each attribute (or attribute group, when they are related), but this results in a very messy Species table, and too many links to the Reference and Project tables, well over the maximum possible in Access. I could split the Species table into smaller tables of related attributes, but I don't think will reduce the overall number of links to the Reference and Project tables greatly, as even within the taxonomic fields there may be multiple sources and references used with the one species. Each species can only appear in the table once. Is there an obvious design change I'm missing here? Any suggestions would be greatly appreciated.