When studying the perturbation theory in quantum field theory (QFT), one cannot restrict oneself to tree diagrams only. Including loops does not change a process type but could lead to divergency of matrix elements. These divergencies arise from the fact that integration over a loop momentum is always fulfilled from zero to infinity. To put it differently, in QFT a natural momentum cut-off is absent. The renormalization theory gives the recipe that allows to systematically extract and eliminate such divergencies from observable quantities. Here, we discuss both general peculiarities of divergent diagrams and possibilities of their regularization. From quantum electrodynamics (QED) it is well known (Section 22.1 of Advanced Particle Physics Volume 1), there is no need to consider all divergent diagrams. It is quite enough to investigate primitive divergent integrals only. To enumerate primitive divergent diagrams corresponds to the fundamental divergencies list, all remaining divergencies being available with the aid of corresponding insertions.