What is Tropospheric Scintillation

5G mobile communication networks are emerging in order to cover the extreme needs for high data rates for delivering multimedia data to mobile communication users. The required bandwidth may be found if millimeter wave bands are fully employed for the establishment of such cellular systems. In this Book Chapter the propagation issues for 5G mobile communication systems are rigorously analyzed and presented. Firstly, the most popular scenarios and architectures of the next generation mobile systems are described and the channel models utilized for the evaluation of physical layer techniques (air interface are given. Secondly, the channel effects are described, i.e. effects due to local environment of the transceivers, atmospheric effects and their combination and the state of the art on the modeling of these effects is analytically presented. Finally, future directions for the propagation and channel model prediction for the next generation mobile communication systems -5G millimeter wave cellular systems are analyzed. Open issues and technical challenges of millimeter wave (mmWave) 5G cellular systems are finally described.