This paper presents Visual Constraint Diagrams (VCD?s), an extension to UML object diagrams for expressing constraints over object models. VCD?s allow 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?s offer 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; 2) VCD?s can be checked at runtime, increasing the value of design documents to developers. An editor and a checker for VCD?s have been implemented as part of the Rosetta software design tool.