In this paper, I present a view of the relative roles of logical presupposition and shared knowledge in it-cleft; constructions. I will argue for a view of it-clefts in which presupposition on the one hand, and indicators of shared information on the other, are understood to have separate functions: indicators of shared knowledge, including prosody, are argued to relate to a speaker's assumptions about the state of the hearer's knowledge and attention, while presuppositions generated on the basis of syntactic form are argued to indicate a speaker's requirements for what should be included within the hearer's discourse model. In the light of this view, I review three common assumptions about clefts relating in particular to the interrelationship of logical presupposition, shared knowledge and prosody, and show how the view argued for here gives an improved analysis of some previously problematic examples. Finally, I make some suggestions regarding what a strict separation between logical presupposition and shared knowledge might imply for psychological accounts of how it-clefts are processed.