Video services are becoming an integral part of future communication systems. Especially for the upcoming 3G wireless networks such as UMTS, video may very well turn out to be the key value addition that achieves the required return of investment. While previous generations of wireless communication systems were primarily designed and used for voice services, next generation systems have to support a broad range of applications in a wide variety of settings. Novel wireless applications such as telemetric and fleet management have introduced wireless networking to the enterprise domain. At the same time the private market sector is booming with the availability of lowâ€œpriced wireless equipment. The early market stages were characterized by the needs of early adopters, mostly for professional use. As the market matures from the early adopters to normal users, new services will be demanded. These demands will likely converge toward the demands that exist for wired telecommunications services. These demands, also referred to as the usual suspects, are comprised of a variety of different services â€ including Internet access for browsing, chatting, and gaming. In addition, entertainment services such as television, cableâ€œTV, and payâ€œper view movies are demanded. With the availability of wireless services, the location is no longer of importance to private users. Thus, the demand of mobile users, connected over wireless networks will approach this mixture of services. With the omnipresence of wireless services, the usage schemes will become independent from location and connection type. One example for such a wireless service is mobile gaming. Private users, who are waiting at an airport or elsewhere, are able to join Internetâ€œbased multi-player games to bridge time gaps. Among the different entertainment services, mobile video will likely account for a large portion of the entertainment services, as are cinemas, video rentals, and television now. This application scenario covers wireless entertainment broadcasting and video on demand. A second, professional application of video in the wireless domain is telemedicine, where remote specialists are enabled to respond to emergencies. The wide area of video services in wireless environments as well as the expectations for wireless communication systems call for an understanding of the basic principles of wireless video streaming. 7 Generally, the video delivered to wireless users is either (i) live video, e.g., the live coverage of a sporting event, concert, or conference, or (ii) prerecorded (stored) video, e.g., a TV show, entertainment movie, or instructional video. Some videos fall in both categories. For instance, in a typical distance learning system, the lecture video is available to distance learners live, i.e., while the lecture is ongoing, and also as stored video, i.e., distance learners can request the lecture video later in the day or week from a video server. In general, there are two ways to deliver video over a packet-switched network (including packet-oriented wireless networks): (i) file download, or (ii) streaming. With file download the entire video is downloaded to the userâ„¢s terminal before the playback commences. The video file is downloaded with a conventional reliable transport protocol, such as TCP. The advantage of file download is that it is relatively simple and ensures a high video quality. This is because losses on the wireless links are remedied by the reliable transport protocol and the playâ€œout does not commence until the video file is downloaded completely and without errors. The drawback of file download is the large response time, typically referred to as startâ€œup delay. The startâ€œup delay is the time from when the user requests the video until playback commences. Especially for large video files and small bandwidth wireless links, the start-up delay can be very large. With video streaming, on the other hand, playback commences before the entire file is downloaded to the userâ„¢s terminal. In video streaming typically only a small part of the video ranging from a few video frames to several hundreds or thousands of frames (corresponding to video play back durations on the order of hundreds of milliseconds to several seconds or minutes) are downloaded before the streaming commences. The remaining part of the video is transmitted to the user while the video playback is in progress. One of the key tradeâ€œoffs in video streaming is between the start-up delay and the video quality. That is, the smaller the amount of the video that is downloaded before streaming commences, the more the continuous video playback relies on the timely delivery of the remaining video over the unreliable wireless links. The errors on the wireless links may compromise the quality of the delivered video in that only basic low quality (and low bit rate) video frames are delivered or some video frames are skipped 8 entirely. Thus, video streaming gives the user shorter start-up delays at the expense of reduced video quality. The challenge of video streaming lies in keeping the quality degradation to a level that is hardly noticeable or tolerable while utilizing the wireless resources efficiently (i.e., supporting as many simultaneous streams as possible). We note that file download and streaming with some start-up delay are suitable only for prerecorded video. The delivery of live video, on the other hand, requires an extreme form of video streaming with essentially no pre-playback download. Another consideration in the delivery of prerecorded video is user interaction (i.e., VCR functions such as fast forward, pause and rewind). Some of these interactions may result in a new start-up delay in video streaming. In this book chapter we introduce video streaming in wireless environments. We first give an introduction to the world of digital video, and outline the fundamental need for compression. Next, we introduce the basics of video compression. We show how video compression is achieved by exploiting different types of redundancies in the raw (uncompressed) video stream. We do this by following one sample video sequence on its way from the raw video to the compressed video stream. This sample video sequence is called Highway, and is publicly available . Next, we introduce the characteristics of the wireless communication channel and study the impact of the wireless link errors on the video quality. Finally, we discuss the different protocols and adaption techniques for streaming the video content over the wireless channel. The model of a transmission chain of a general wireless communication system for video streaming is given in Figure 1. At the sender side the video source is passed to the video (source) encoder. The compressed video stream is passed to the transport process which in turn passes the stream plus some overhead information to the channel coder and modulation part for transmission over the wireless link. At the receiver side the process is reversed.