Roos H. On the Problem of Defining Local Thermodynamic Equilibrium (32K, LaTeX) ABSTRACT. The concept of local thermodynamic equilibrium is up to now defined only on a phenomenological level, there is no definition in the context of rigorous Statistical Mechanics. The paper does not give am answer, it aimes at bringing the related problems into sharp focus. A possible ansatz using a scaling limit is discussed. The central thesis is the following: Thermodynamics necessitates two ``hypotheses on thermalization": a first one to describe global equilibrium, i. e., to single out a set of states which are to be interpreted as equilibrium states; and a second one in order to enforce local equilibrium.