K-frames, as a new generalization of frames, have important applications, especially in sampling theory, to help us to reconstruct elements from a range of a bounded linear operator K in a separable Hilbert space. In this paper, we focus on the reconstruction formulae to characterize all K-duals of a given K-frame. Also, we give several approaches for constructing K-frames.2010 AMS Mathematics subject classification. Primary 42C15, Secondary 41A58.