  We present a general formalism for charecterizing 2-time quantum states,
describing pre- and post-selected quantum systems. The most general 2-time
state is characterized by a `density vector' that is independent of
measurements performed between the preparation and post-selection. We provide a
method for performing tomography of an unknown 2-time density vector. This
procedure, which cannot be implemented by weak or projective measurements,
brings new insight to the fundamental role played by Kraus operators in quantum
measurements. Finally, after showing that general states and measurements are
isomorphic, we show that any measurement on a 2-time state can be mapped to a
measurement on a preselected bipartite state.
