  In these lectures we give a concise introduction to the ideas of renormalon
calculus in QED and QCD. We focus in particular on the example of the Adler D
function of vacuum polarization, and on relations between perturbative
renormalon ambiguities and corresponding non-perturbative Operator Product
Expansion (OPE) ambiguities. Recent work on infrared freezing of Euclidean
observables is also discussed.
