In this section we first examine the nature of applications using digital audio and video streams and then proceed to show how the characteristics of these continuous media impose requirements for the underlying communications network. Traditionally, these media were carried by circuit switched networks with fixed bandwidth allocations and constant transmission delays, while packet switched networks were developed to accommodate data sources with bursty traffic, ignoring timing issues. This change of paradigm raises many issues for packet network technology. Hierarchical coding is a technique especially suited to continuous media that can be used to deal with many of these issues, and is also examined here.