We propose two new methods for the definition of integrity constraints in object-oriented conceptual modeling languages. The first method applies to static constraints, and consists in representing them by special operations, that we call constraint operations. The specification of these operations is then the definition of the corresponding constraints. The second method, which is a slight variant of the previous one, applies to creation-time constraints, a particular class of temporal constraints. Both methods allow the specialization of constraints and the definition of exceptions. We include also an adaptation of the two methods to the UML.