Millimeter wave (mmW) frequencies between 30 and 300 GHz offer significantly greater bandwidths than current cellular networks, enabling higher capacity through beamforming and spatial multiplexing. This paper surveys measurements and capacity studies to assess mmW technology, focusing on small cell deployments in urban environments. Measurements in New York City at 28 and 73 GHz show that mmW systems can provide significant outdoor coverage up to 200 m from a low-power base station, with capacity gains over 4G networks. However, mmW systems require significant redesign due to challenges like directional transmission, isolation between links, and outage risks. Technologies such as adaptive beamforming, multihop relaying, and heterogeneous networks can address these challenges. mmW signals are vulnerable to shadowing and intermittent connectivity, but their small wavelengths and advanced antenna arrays enable high gain and directionality. mmW systems can offer orders of magnitude higher capacity than current networks, but their performance is limited by NLOS propagation and cell edge users. The paper also discusses the potential of mmW systems in urban environments, highlighting the need for heterogeneous networks and the importance of spatial multiplexing. The results show that mmW systems can provide high capacity and improved performance, but their deployment requires careful design and optimization.Millimeter wave (mmW) frequencies between 30 and 300 GHz offer significantly greater bandwidths than current cellular networks, enabling higher capacity through beamforming and spatial multiplexing. This paper surveys measurements and capacity studies to assess mmW technology, focusing on small cell deployments in urban environments. Measurements in New York City at 28 and 73 GHz show that mmW systems can provide significant outdoor coverage up to 200 m from a low-power base station, with capacity gains over 4G networks. However, mmW systems require significant redesign due to challenges like directional transmission, isolation between links, and outage risks. Technologies such as adaptive beamforming, multihop relaying, and heterogeneous networks can address these challenges. mmW signals are vulnerable to shadowing and intermittent connectivity, but their small wavelengths and advanced antenna arrays enable high gain and directionality. mmW systems can offer orders of magnitude higher capacity than current networks, but their performance is limited by NLOS propagation and cell edge users. The paper also discusses the potential of mmW systems in urban environments, highlighting the need for heterogeneous networks and the importance of spatial multiplexing. The results show that mmW systems can provide high capacity and improved performance, but their deployment requires careful design and optimization.