The invention relates to a method and a system for defining structures of object and/or data models (OM), comprising at least one schema (XS1, XS2) for describing the structures. An upward and downward compatible schema evolution is achieved in that a labeling of a version of the corresponding schema (XS1, XS2) is carried out in a first attribute (10, 20) of a schema (XS1, XS2), wherein the namespace (1) used in the corresponding schema (XS1, XS2) and the type and element name (11a..14a, 21a..24a) used in the corresponding schema (XS1, XS2) are preserved regardless of the version, wherein the types and elements (11..14, 21..24) are expanded only while preserving the type or element name (11a..14a, 21a..24a). Unexpanded types and elements (21..24) are accepted in the schemas (XS2) of a newer version unchanged from the types or elements (11..14) of schemas (XS1) of an older version.