 An ever hot topic, quality of service is a very important concern in every network, particularly in next generation networks. In couple of modules, we are going to explore quality of service and quality of experience which is a related terminology for next generation networks. In fact, there are numerous books which address quality of service as a topic. The breadth and scope of quality of service is so huge that it needs to be summarized in these lectures. I will try my best to keep it focused and crisp. We just look at the requirements which define the necessary background for discussing the topic. The NGNs are primarily packet-based networks. Since the primary purpose is to have the telecom services on the next generation network architecture, the broadband access provides the necessary underlying telecom support. Using quality of service enabled transport technologies, NGN is well poised to provide the public switch telecom networks, public land mobile networks, broadcast network services, all unified. These networks had no problem of providing quality of service because each network was designed to assure the quality of service for a particular service type. However, this is not the case in NGN, so we are expecting NGN to really provide some kind of intricate and well-defined architecture that is able to provide QS for all these network and service types. NGN is actually highlighted or underpinned for its fully IP design. It means it is offering heterogeneous services and each service is integrated into a unified network. In addition to these well-known classical services like television, PSTN, PLMN etc., the internet based services such as web email browsing etc., are also to be offered as best effort services which are going to coexist with QS specific services. Now the design of NGN QOS mechanism has to be so that it provides the same experience regardless of the service type across the users and the services. The views through which quality of service can be interpreted is quite diverse. In terms of classical understanding, the traffic engineering as a quality of service mechanism is defined as a measurable method that allows certain parameters to be measured and controlled. So it means the network parameters are actually selected and measured using certain techniques. Through this the quality of service can be offered, controlled and guaranteed. There is another view by the ITUT, actually it is known as the ITUT 800 document. It looks at the quality of service in totality. It means it specifies the characteristics of the telecom services which are either explicated or are assumed to be there, that is, they are implied as the characteristics of services which are to be provided at a certain level to the users. Then there is a network service provider's view as well, that is, a service provider or a network provider should actually provide service quality by segmenting the traffic into different streams on some basis and then differentiating between these service types. Once a differentiation mechanism exists, then each traffic type can be treated differently. In this way some would be the privileged traffic types and some would be the best effort traffic type. Then finally the user's viewpoint. The user actually is the one that is using the service at the end. The user satisfaction is central to the overall quality of service perspective. However, since the users have a subjective opinion or highly personalized opinion on their definition of quality of service, it is not exclusively defined because it is their perception which is determining if a service is good or bad. So with these views, we are now in a position to move on and explore the other details of the quality of service.