The two-pion exchange contributions to nucleon-nucleon forces are calculated from our knowledge of pion-nucleon phase shifts and pion-pion interaction which are used as input into dispersion relations with the subtractions required by Regge asymptotic behavior. At low energies. nucleon- nucleon potentials are derived, which, with the addition of pi -meson and omega -meson exchange, explain many features of the phenomenological potentials without any adjustable parameter. However, the calculated spin-orbit and central potentials have significant nonlocal components at small distances. (auth)