The connected world paradigm effectuated through the proliferation of mobile devices, Internet of Things (IoT), and the metaverse will offer novel services in the coming years that need anytime, anywhere, high-speed access. The success of this paradigm will highly depend on the ability of the devices to always obtain the optimal network connectivity for an application and on the seamless mobility of the devices. This book will discuss 6G concepts and architectures to support next-generation applications such as IoT, multiband devices, and high-speed mobile applications. IoT applications put forth significant challenges on the network in terms of spectrum utilization, latency, energy efficiency, large number of users, and supporting different application characteristics in terms of reliability, data rate, and latency.While the 5G network developmentwas motivated by the need for larger bandwidth and higher quality of service (QoS), 6G considerations are supporting many users with a wide application requirement, lowering network operating cost, and enhanced network flexibility. Network generations beyond 5G are expected to accommodate massive number of devices with the proliferation of connected devices concept in connected cars, industrial automation, medical devices, and consumer devices. This book will address the fundamental design consideration for 6G networks and beyond. There are many technical challenges that need to be explored in the next generation of networks, such as increased spectrum utilization, lower latency, higher data rates, accommodating more users, heterogeneous wireless connectivity, distributed algorithms, and device-centric connectivity due to diversified mobile environments and IoT application characteristics. Since 6G is a multidisciplinary topic, this book will primarily focus on aspects of device characteristics, wireless heterogeneity, traffic engineering, device-centric connectivity, and smartness of application.