This chapter discusses Quality of Service (QoS) and the related subject of traffic engineering. QoS is becoming increasingly important for both private intranets and the Internet and is beginning to have a fundamental affect on the way we design networks, particularly large public networks like the Internet. It is notable that the long-term requirement for QoS mechanisms is a hotly debated topic. It is proposed in some quarters that the increased use of fiber optics, together with Wavelength Division Multiplexing (WDM), will make bandwidth so abundant and inexpensive in the future that QoS will be redundant. Unfortunately, experience tells us that these benefits rarely get passed on to users quickly, if at all, since many service providers still operate virtual monopolies in many regions of the world. We are likely to require more QoS features for the foreseeable future. Although this chapter is concerned with service quality, in reality most IP internetworks today provide only one class of service, called best effort (which essentially means no guarantees for delivery, bandwidth, or response times). In effect, all traffic has the same priority. Over the last decade use of the Internet has increased dramatically for both private and business users. The commercial use of the Internet in particular is a key driver for the implementation of end-to-end service guarantees for e-commerce and business-critical applications; this requires classification and special handling of packet streams at the session or flow level. Businesses are beginning to require tighter Service-Level Agreements (SLAs) from service providers...