The World Wide Web is evolving from being a pure information repository to a more functional and service oriented platform thanks to technologies such as Web Services. This technology offers a homogeneous representation of Web elements and the ways they are communicating that make it possible to deal with the inherent structural and behavioural heterogeneities of the Web. A Web service can be seen as an autonomous functional element that is loosely coupled to other Web services and can be discovered and deployed in Web-based applications. Autonomity and loose coupling make Web services a viable light weight complementary componentbased approach for design and development of dynamic distributed systems for more heavy weight solutions such as OMG's CORBA and Microsoft's DCOM. In this paper, we take the position that if Web Services are going to be considered as reusable commercial of-the-shelf (COTS) components, their Quality of Service (QoS) needs to be expressed explicitly and measured independently. More specifically, we present and discuss possible quality aspects that need to be represented and quantified for Web Services.