This paper discusses the key challenges and technologies for 5G, emphasizing that it will not be an incremental improvement over 4G but a major paradigm shift involving high carrier frequencies, massive bandwidths, and dense base station and device networks. 5G will be highly integrative, combining new air interfaces with LTE and WiFi for universal coverage. The core network will need unprecedented flexibility and intelligence, and spectrum regulation will need to be rethought. Energy and cost efficiency will be critical. The paper identifies key research challenges and preliminary standardization activities, and provides a comprehensive overview of current literature. The paper discusses the engineering requirements for 5G, including data rate, latency, energy and cost efficiency, and device types and quantities. 5G will need to support a much larger and more diverse set of devices, including a large number of low-rate devices in addition to traditional high-rate mobile users. This will require significant changes to the control plane and network management. The paper then discusses key technologies for achieving the 1000x data rate increase, including extreme densification and offloading, increased bandwidth, and increased spectral efficiency. The paper also discusses challenges related to extreme densification, multi-RAT association, mobility support, cost, and the integration of mmWave and massive MIMO technologies. The paper concludes that 5G will be a major technological advancement that will require significant research and development efforts.This paper discusses the key challenges and technologies for 5G, emphasizing that it will not be an incremental improvement over 4G but a major paradigm shift involving high carrier frequencies, massive bandwidths, and dense base station and device networks. 5G will be highly integrative, combining new air interfaces with LTE and WiFi for universal coverage. The core network will need unprecedented flexibility and intelligence, and spectrum regulation will need to be rethought. Energy and cost efficiency will be critical. The paper identifies key research challenges and preliminary standardization activities, and provides a comprehensive overview of current literature. The paper discusses the engineering requirements for 5G, including data rate, latency, energy and cost efficiency, and device types and quantities. 5G will need to support a much larger and more diverse set of devices, including a large number of low-rate devices in addition to traditional high-rate mobile users. This will require significant changes to the control plane and network management. The paper then discusses key technologies for achieving the 1000x data rate increase, including extreme densification and offloading, increased bandwidth, and increased spectral efficiency. The paper also discusses challenges related to extreme densification, multi-RAT association, mobility support, cost, and the integration of mmWave and massive MIMO technologies. The paper concludes that 5G will be a major technological advancement that will require significant research and development efforts.