It is shown that there are two inherent Aharonov - Bohm effects in Magnetohydrodynamics, in each case a magnetic flux induces a "phase" on quantities that do not come under the influence of the magnetic field directly. Those quantities include the velocity fields and "external" magnetic field. Those phases quantify two well known Topological conservation laws of the magnetic and cross helicities. The above phases are also useful for introducing a very efficient variational principle for magnetohydrodynamics which is given in terms of only four independent functions for non-stationary flows. This is less than the seven variables which appear in the standard equations of magnetohydrodynamics which are the magnetic vector field B the velocity vector field v and the density tho.