This chapter focuses on the core video encoding and decoding technologies used in XR services like Meta Quest 2 and Microsoft HoloLens 2 and the popular over-the-top (OTT) video services like Netflix, Disney+, and YouTube. The sections cover the history of Skype (as it is one of the earliest video conferencing and Voice over IP (VoIP) applications in the world) and YouTube and the evolution of their video and audio codec technologies. In addition, details of the ...