Renormalization of mass of an electron inside the crystal In Cheng and Li's book, Gauge theory of Elementary Particle Physics, he essentially says that renormalization has nothing to do with infinities. Even in a totally finite theory, we would still have to renormalize physical quantities. For example, the mass $m^*$ of an electron inside the crystal is renormalized from the mass $m$ it has outside the crystal (due to the interaction inside the crystal). However, unlike relativistic QFT, both $m$ and $m^*$ are measurable and finite. Therefore, the correction $\delta m=m-m^*$ should also be finite. 
How does one calculate this correction $\delta m$? If one uses quantum field theory, he finds that the correction to electron mass is logarithmically divergent.
 A: The theory of a solid body is the typical example of an effective field theory, typically the finite-temperature. Therefore, all integrals in momentum space are bounded from above by the physical cut-off, while loops, of course, remain. In order to compute the correction to the electron mass in a crystal You need then just to compute the self-energy of the electron by using the finite-temperature Green functions formalism. Of course, there can be a correction proportional to the mass term. 
There are also typical examples from the high-energy effective field theories, like chiral perturbation theory, which describes the pseudo-scalar mesons octet interactions below the scale of the QCD chiral symmetry spontaneous breaking. In this theory the natural cut-off is the proton mass. Without EM interactions the masses $\pi^{0}, \pi^{\pm}$ are equal. However, once the EM interactions are turned on, loop corrections to the charged mesons masses appear. With finite cut-off, these corrections are, of course, finite. 
