We describe the method of QCD sum rules, using calculation of the couplings of the nucleon to axial fields as an illustrative example. Our interest is centered about the behavior of a system of quarks, antiquarks, and gluons under the influence of an external field, leading to a modified version of the method of QCD sum rules. In particular, the isovector axial coupling constant go is determined from a QCD sum rule for (gA-l), and it is shown that, with standard values of the quark condensates, gA=1.26±0.08. A sum rule is also obtained for the ”isoscalar” axial coupling constant gA^s, which is found to be 0.13 if the isovector values of susceptibilities are used.