Software developers spend most of their time modifying and maintaining existing products. This is because systems, and consequently their design, are in perpetual evolution before they die. Nevertheless, dealing with this evolution is a complex task. Before evolving a system, structural modi cations are often required. The goal of this kind of modi cation is to make certain elements more extensible, permitting the addition of new features. However, designers are seldom able to evaluate the impact, on the whole model, of a single modi cation. That is, they cannot precisely verify if a change modi es the behavior of the modeled system. A possible solution for this problem is to provide designers with a set of basic transformations, which can ensure behavior preservation. These transformations, also known as refactorings, can then be used, step by step, to improve the design of the system. In this paper we present a set of refactorings and explain how they can be designed so as to preserve the behavior of a UML model. Some of these refactorings are illustrated with examples.