This dissertation is an inquiry into the identity conditions for propositions, with a view to determining whether a proposition can be conceived of as composed, in different ways, of several different combinations of ultimate constituents or whether the identity conditions for propositions require them each to have a single ultimate analysis. The inquiry is prompted by there being many instances of translation that seem to entail the expression of a single proposition by differently structured sentences. If these sentences reflect alternative (...) ultimate structures of the propositions they express, then some interesting consequences may follow. For example, propositions may not involve ontological presuppositions in the way that they do if they have unique structures. Conversely, grasping a particular proposition may require, not possessing a particular conceptual apparatus, but possessing any one of several. Hence the motivation for inquiring what sort of structure propositions are required to have by their identity conditions. ;Frege's remark that a proposition can be carved up in more than one way suggests that a Fregean criterion for propositional identity would allow propositions to have multiple ultimate structures, and that it might even illuminate the relationship among the structures that a proposition can have. The dissertation accordingly seeks to reconstruct a Fregean criterion for propositional identity, beginning with a Fregean criterion for expressing the same proposition. Most of the conclusions concern the latter kind of criterion, for work on it takes up a large part of the dissertation. It is concluded that Frege provides no criterion that is both noncircular and compatible with a proposition's having more than one ultimate structure. Additional results are a Fregean treatment of indexicals, the discovery that logical equivalence plays a central role in Frege's thinking about propositional identity, and the concomitant discovery that the idea of a criterion of propositional identity based on logical equivalence leads to substantial difficulties. (shrink)