According to Szyperski, "a software component is a unit of composition with contractually specified interfaces and explicit context dependencies only". But it is well known that these contractually specified interfaces should go well beyond mere syntactic aspects: they should also involve functional, synchronization and Quality of Service (QoS) aspects. In large, mission-critical component based systems, it is also particularly important to be able to explicitly relate the QoS contracts attached to provided interfaces with the QoS contracts obtained from required interfaces. In this paper we propose a language called QoSCL (defined as an add-on to the UML2.0 component model) to let the designer explicitly describe and manipulate these higher level contracts and their dependencies.