2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/210354After a brief introduction to Heavy Quark Effective Theory, we discuss $α$ representation in HQET and methods of calculation of some kinds of HQET diagrams up to three loops.Lectures given at II Helmholtz summer school "Heavy quark physics", Dubna, August 2008; v2: extended, new derivations of some results, a few references added; v3: a few typos fixedHigh Energy Physics - Phenomenologytext