2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/48154I show that there is a unique and well behaved derivative expansion of an effective action at finite temperature. The result is true for all formalisms including the popular Closed Time Path and Imaginary Time methods.RevTeX, 4 pages, 3 Feynman diagrams as eps files. Talk given at TFT98 - the 5th Workshop on Thermal Field Theories and their applications, Regensburg, Germany, 10th-14th August 1998High Energy Physics - Phenomenologytext