The IP Multimedia Subsystem (IMS), a new mobile network infrastructure, is a whole new way to deliver multimedia—including voice, video, and data—regardless of the device (such as mobile phone, landline phone, cable, or Internet), or the access medium (such as cellular, WiFi, Broadband, or fixed-line). IMS is poised to change the way all of us relate to our increasingly digital world. This paper serves two main purposes: to explain the basic vision and need for the IMS architecture, and to provide the reader with a high-level understanding of IMS works.