In 1925, with Werner Heisenberg he developed the Kramers–Heisenberg dispersion formula, and in 1926 he was one of the authors of the WKB method. He is also credited with introducing in 1948 the concept of renormalization into quantum field theory, although his approach was nonrelativistic. He is also credited for the Kramers–Kronig relations with Ralph Kronig which are mathematical equations relating real and imaginary parts of complex functions constrained by causality. One further refers to a Kramers turnover when the rate of thermally activated barrier crossing as a function of the damping goes through a maximum, thereby undergoing a transition between the energy diffusion and spatial diffusion regimes. He is also known for Kramers' degeneracy theorem.