**Abstract**

A consistent theory of gravitational redshift in cosmology (GRC) is formulated. The global GRC arises due to weakening of gravitational time dilation due to decreasing of matter density during the propagation time of photons. In the expanding world the local GRC arises due to the weakening of gravity of the sphere between observer and source, since photons emitted at a smaller radius arrive at a larger one. In static world there is no GRC at the exchange of photons at the periphery of this sphere. In any case photons from observer to source have the same GRC as photons from source to observer, which is in agreement with the cosmological principle. Consequences of the local and global GRC for cosmological models and their parameters, as well as corrections to data on distant objects and CMB, are considered. In Appendix the inconsistency of two former treatments of the gravitational frequency shift in cosmology is shown. They: a) did not take into account the global GRC b) derived the local GRC not from the field of the sphere between the source and observer, but from the field of spheres around one of them c) contradicted each other (the signs of shifts are opposite) d) violated cosmological principle (changing the propagation direction changes the sign of shift) and e) were based on the delusion that the Friedmann model supposedly contains the gravitational shift.