The one-fluid approach to the theory of viscous-resistive ballooning modes in a Tokamak is developed. Qualitative ideas about essential viscous effects (inertia renormalization and viscous resistive effect) are given. A procedure for simplifying the MHD equations in a resistive layer, which takes into account viscosity and other typical effects, is described. The weakly ballooning approximation is used for averaging the ballooning equations. The equations obtained are a generalization of a number of different particular cases of ballooning equations obtained by previous authors.