This is the first of a series of papers devoted to a systematic study
of QCD correlation functions. Our aim is to reach a quantitative under
standing of the forces between quarks in all channels, and, in particu
lar, the role played by instanton-induced effects. The topic of this f
irst paper is to work out approximate formulae for quark propagators i
n a multi-instanton environment, which include the 't Hooft interactio
n to all orders. As an application, we use the simplest possible model
of the kind, the so-called ''random instanton liquid model'' (RILM).
The results obtained can also be related to properties of heavy-light
mesons, and those are found to be consistent with phenomenology.