Quality of Service (QoS) is one of major issues faced today by the designers of modern comunications networks. Unlike legacy networks which typically dealt with single homoegenous application (e.g., the telephony network) advanced networks attempt at accomodating a large variety of heterogenous applications, such as phone conversations, live video, streaming video, Web, email, FTP, and others. The great differentiation in the properties of these applications places serious challenges to network designers. We will explore the QoS mechanisms as designed recent technologies for data transfer in IP mode.Syllabus outline is as follows: Class of service and QoS, IP based traffic, Integrated and differentiated services and associated protocols for QoSs. ATM: basic concepts and technology, QoS and classes of service, policing and shaping, scheduling: fair queueing, virtual paths and virtual connections; QoS for Frame Relay & MPLS; Call Admission Control (CAC): CAC for CBR traffic, CAC for VBR traffic; QoS routing; Internet communications and QoS in the internet; Web Performance: caching and pre-fetching, load balancing, distributed load balancing and contents puching; QoS for mobile communications.