The transmission line tower foundations are executed following extensive paths, where different structural foundation conditions are found, in reason of natural horizontal ground layer variations. Then, one may alternatively to schedule and to execute special and specific studies or, conversely, to substantially reduce the geothecnical investigation, with sensible economical loses in the transmission line design and behavior. In such way, a methodology is proposed to evaluate conveniently the site according to its geology, orography , hydrography and pedology as well to related atmospheric local manifestations, able to submit severe mechanical solicitations to the towers and their foundations. In this way, one proposes a methodology to govern the study of these foundations, considering the tower x site x foundation relations and searching for convenient technical and acceptable economical solutions. Guyed and self-supported models are considered for towers, and the foundations are represented by footings, as the superficial type, and piers, as the deep type. The rio do Prata basin is chosen to constitute the large scenery of the study, because its abundant available data concerned to already operational transmission lines, or still under design and construction ones. Moreover, seeking the improvement of the tower foundation design a study based on structural reliability is developed considering some foundation models selected as mentioned above. Foundation failure probabilities are determined by the FERUM computational program and the risk of failure is obtained from economical damages caused by foundation failures. In the last, a parametric study is developed to evaluate how does the failure probability behave according to variations in the foundation soil, wind velocity and footing dimensions and depth.