Effective methods in QCD and the phenomenology of hadrons

To place the problem in perspective I will first discuss the decoupling of heavy quarks in QCD which is a simpler perturbative problem. Then, I will review two experimental observables (the σ-term in πN scattering and the polarised deep inelastic scattering) which diagnose the possibility of non-perturbative effects associated with the decoupling of the strange quark and will next discuss their possible interpretation on the basis of the skyrme model. I will also present some simple-minded results for a related low-energy application to the meson-nucleon scattering lengths obtained in a chiral effective lagrangian approach