The Unified Modeling Language (UML) is a set of notations for modelling object-oriented systems. It has become the de facto standard. Most of its notations are diagrammatic. An exception to this is the Object Constraint Language (OCL) which is essentially a textual, stylised form of first order predicate logic. We describe a notation, constraint diagrams, which were introduced as a visual technique intended to be used in conjunction with the UML for object-oriented modelling. Constraint diagrams provide a diagrammatic notation for expressing constraints (e.g., invariants) that could only be expressed in UML using OCL. Keywords Modelling, visual formalism, object-oriented software development, formal methods.