Raw digital video signals are very large in size, making it very difficult to transmit or store them. Video compression techniques are therefore essential enabling technologies for digital multimedia applications. Since 1984, a wide range of digital video codecs have been standardized, each of which represents a step forward either in terms of compression efficiency or in functionality. This chapter describes the basic principles behind most standard block-based video codecs currently in use. It begins with a discussion of the types of redundancy present in most video signals (Section 2.2) and proceeds to describe some basic techniques for removing such redundancies (Section 2.3). Section 2.4 investigates enhancements to the basic techniques which have been used in recent video coding standards to provide improvements in video quality. This section also discusses the effects of communication channel errors on decoded video quality. Section 2.5 provides a summary of the available video coding standards and describes some of the key differences between them. Section 2.6 gives an overview of how video quality can be assessed. It includes a description of objective and subjective assessment techniques.