The Access to Biological Collections Data (ABCD) Schema is an evolving comprehensive standard for the access to and exchange of data about specimens and observations (a.k.a. primary biodiversity data).The ABCD Schema attempts to be comprehensive and highly structured, supporting data from a wide variety of databases. It is compatible with several existing data standards. Parallel structures exist so that either (or both) atomised data and free-text can be accommodated. Version 1.2 is currently in use with the GBIF (Global Biodiversity Information Facility) and BioCASE (Biological Collection Access Service for Europe) networks.Apart from the GBIF and BioCASE networks, the potential for the application of ABCD extends to internal networks, or in-house legacy data access (e.g. datasets from external sources that shall not be converted and integrated into an institution' s own data, but be kept separately, though easily accessible). By defining relations between terms, ABCD is a step towards an ontology for biological collections.