  We examine the two-photon exchange corrections to elastic electron-proton
scattering within a dispersive approach, including contributions from both
Nucleon and Delta intermediate states. The dispersive analysis avoids off-shell
uncertainties inherent in traditional approaches based on direct evaluation of
loop diagrams, and guarantees the correct unitary behavior in the high energy
limit. Using empirical information on the electromagnetic nucleon elastic and
N-Delta transition form factors, we compute the two-photon exchange corrections
both algebraically and numerically. Results are compared with recent
measurements of e+p to e-p cross section ratios from the CLAS, VEPP-3 and
OLYMPUS experiments, as well as with polarization transfer observables.
