Through the last few years, we have qualified an overriding explosion of the Internet that penetrated all aspects of everyday life. Starting off as a purely academic research network, the Internet is now extensively used for educational, entertainment and as a very promising and dynamic marketplace, and is envisioned to evolve into a vehicle of true collaboration and a multi-purpose working environment. Although based on a best effort service model, the simplicity of its packet-switched nature and the flexibility of its underlying packet forwarding rule, IP, accommodate millions of users while offering acceptable performance. In the meantime, new exciting applications and networked services have emerged, posing more stringent demands for the network. In order to offer a better than best-effort Internet, new service models that offer certain performance guarantees to applications have been proposed. While several of these proposals are in place and many QoS-enabled networks are operating, there is still a lack of comprehension about the precise requirements new applications have in order to function with high or accepted levels of quality. Furthermore, what is required is an understanding of how network-level QoS reflects on the actual application utility and usability. This document tries to fill this gap, by presenting an extensive survey on applications' QoS needs. It identifies applications that cannot be accommodated by today's modest best-effort Internet service model and reviews the nature of these applications as far as their behavior with respect to the network is concerned. It presents guidelines and recommendations on what levels of network performance parameters are expected so that applications operate with high quality, or, within ranges of acceptable quality. This thesis highlights the role of the application and application developers in achieving the expected performance from a network service. It argues that the network itself cannot guarantee good performance unless assisted by well-designed applications that can tailor their behavior, by employing suitable adaptation mechanisms, to whatever network conditions or service model is present. At its present form, this thesis is primarily focused on audio and video applications. It presents a detailed analysis of the end-to-end performance requirements of applications like audio-video conferencing, voice over IP and streaming of high quality audio and video, and overviews what adaptation choices are available to these applications so that they can operate within a wider range of network conditions.