ConceptBase.cc is a multi-user deductive database system with an object-centered data model. Its ability to represent information at any abstraction level (data, class, metaclass, meta-metaclass) makes it a powerful tool for metamodeling and engineering of customized modeling languages. The system is accompanied by a highly configurable graphical user interface that builds upon the logic-based features of the ConceptBase.cc server. ConceptBase.cc can represent information at the data level (example data, traces of process executions), the class level (schemas, process definitions), the metaclass level (constructs of modeling languages), the meta-metaclass level (constructs for defining modeling languages), and so forth. Uniform object representation. All objects are represented in a uniform quadruple data structure called P-facts pioneered by the developers of the Telos language. Objects, their attributes, specializations, and instantiations are all represented as P-facts. By this, for example, attributes of objects can have attributes and are instances of other attributes. Even specialization between attributes is fully supported. Logical expressions. Rules, constraints, and queries are expressed in first-order logic formulas. Internally, the system transforms them into Horn clauses interpreted by a Datalog-based evaluation machine. Logical expressions in ConceptBase.cc can range over any object regardless of its type (node vs. link) and its abstraction level. Datalog is known for being the most robust computational system for evaluating logical expressions. Active rules. Active rules update the database or call external routines as a reaction to events.