This paper is concerned with the combinatory restrictions on object clitics in a selection of Romance languages and in Modern Greek. As will be shown, when clitics combine with other clitics their behavior differs from their behavior in isolation. With clitic combinations, we can observe three problematic phenomena: deviations from their phonological form, substitution of clitics, and the unexpected non-occurrence of particular clitic , combinations. We can further observe that the phonological and morphological contexts in which these restrictions apply are strikingly similar in the languages examined. I argue that an Optimality Theoretic approach provides the most promising means to analyse these data since such an approach can handle interactions of concurring requirements from different linguistic modules as well as explain the variation between the languages.The paper is organized as follows. First, I show the properties of object clitic combinations. I argue in favor of considering the combination of object clitics as an independent morphological unit, namely the Clitic Sequence. Thus, it is possible to refer to the Clitic Sequence with phonological and morphological wellformedness constraints. In section 3, I present these constraints together with some theoretical background assumptions on the linking mechanism I assume. Following this, I show in section 4 how the language specific realizations of the clitic combinations come about by the language specific rankings of the constraints. Finally, these rankings are compared and some predictions are derived about general ranking tendencies of constraints which belong to different linguistic modules.