This paper explores the potential and challenges of millimeter wave (mmW) cellular networks, focusing on small cell deployments in urban environments. mmW frequencies between 30 and 300 GHz offer significantly greater bandwidths and gains through beamforming and spatial multiplexing. Measurements in New York City at 28 and 73 GHz demonstrate that significant non-line-of-sight (NLOS) outdoor coverage is possible up to about 200 meters from a low-power base station. mmW systems can provide more than an order of magnitude increase in capacity over current 4G networks at similar cell densities. However, significant redesign is required for cellular systems to fully realize these gains, including adaptations in multiple access, channel structure, synchronization, and receiver design. The paper discusses various technologies such as adaptive beamforming, multi-hop relaying, heterogeneous network architectures, and carrier aggregation to address these challenges. The authors also present detailed channel measurement results and statistical models, showing that mmW systems can achieve high capacity and spectral efficiency, but face issues like shadowing and rapid channel fluctuations. The paper concludes with a discussion on the design implications and potential solutions for mmW cellular networks.This paper explores the potential and challenges of millimeter wave (mmW) cellular networks, focusing on small cell deployments in urban environments. mmW frequencies between 30 and 300 GHz offer significantly greater bandwidths and gains through beamforming and spatial multiplexing. Measurements in New York City at 28 and 73 GHz demonstrate that significant non-line-of-sight (NLOS) outdoor coverage is possible up to about 200 meters from a low-power base station. mmW systems can provide more than an order of magnitude increase in capacity over current 4G networks at similar cell densities. However, significant redesign is required for cellular systems to fully realize these gains, including adaptations in multiple access, channel structure, synchronization, and receiver design. The paper discusses various technologies such as adaptive beamforming, multi-hop relaying, heterogeneous network architectures, and carrier aggregation to address these challenges. The authors also present detailed channel measurement results and statistical models, showing that mmW systems can achieve high capacity and spectral efficiency, but face issues like shadowing and rapid channel fluctuations. The paper concludes with a discussion on the design implications and potential solutions for mmW cellular networks.