  We present here a relation of different types of Friedrichs models and their
use in the description and comprehension of resonance phenomena. We first
discuss the basic Friedrichs model and obtain its resonance in the case that
this is simple or doubly degenerated. Next, we discuss the model with $N$
levels and show how the probability amplitude has an oscillatory behavior. Two
generalizations of the Friedrichs model are suitable to introduce resonance
behavior in quantum field theory. We also discuss a discrete version of the
Friedrichs model and also a resonant interaction between two systems both with
continuous spectrum. In an Appendix, we review the mathematics of rigged
Hilbert spaces.
