This paper presents visual constraint diagrams (VCD), an extension to UML (Unified Modeling Language) object diagrams for expressing constraints over object models. VCD allows designers to express well-formedness constraints that cannot be expressed using class diagrams alone; an example of such a constraint is that a linked list data structure cannot have any loops. VCD offers two advances over existing techniques: (1) they allow constraints to be expressed within the visual notation of UML, without resorting to complex textual notations such as OCL; and (2) VCD can be checked at runtime, increasing the value of design documents to developers. An editor and a checker for VCD have been implemented as part of the Rosetta software design tool.