Second-Order Principles in Specification Languages for Object-Oriented Programs Bernhard Beckert, Kerry Trentelman Within the setting of object-oriented program specification and verification, pointers and object references can be considered as relations between the elements of a data structure. When we specify properties of these data structures, we often describe properties of relations. Hence it is important to be able to talk about relations and their properties when specifying object-oriented programs or programs with pointers. Many interesting properties of relations such as transitive closure, finiteness, and generatedness are not expressible in first-order logic (FOL); hence neither are they expressible in first-order fragments of specification languages. In this paper we give an overview of the different ways such properties can be expressed in various logics, with a particular emphasis on extensions of FOL, i.e. transitive closure logic, fixed-point logic, and first-order dynamic logic. Within the paper we also discuss which of these extensions already are - or in fact should be - implemented within specification languages. We feel that such a discussion is necessary since it is often the case that when an extension of FOL is implemented within a specification language it is done so in an ad hoc manner.