2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/1905721. QCD Sum Rules: 20 Years After; 2. QCD Vacuum and Basics of the SVZ Method: 2.1 General ideas; 2.2. Getting started/Playing with toy models; 3. Vacuum Condensates; 4. Rho Meson in QCD; 5. Basic Theoretical Instrument -- Wilson's OPE; 6. Practical Version of OPE; 7. Low Energy Theorems; 8. Are All Hadrons Alike? 9. Ecological Niche; 10. New Developments: 10.1 Light-cone sum rules; 10.2 Heavy flavor sum rules; 11. Sum Rules and Lattices; 12. Vacuum Fluctuations are Subtle Creatures; 13. Instead of Conclusions.Lecture given at the 1997 Yukawa International Seminar Non-Perturbative QCD -- Structure of the QCD Vacuum, Kyoto, December 2 -- 12, 1997. 80 pages, LATEX, 23 eps figuresHigh Energy Physics - Phenomenologytext