Radiowaves at frequencies at VHF and above are capable of penetrating the ionosphere, and therefore provide transionospheric communications. For transionospheric communications, the most significant degradations due to background ionization include Faraday Rotation and Group Delay. These degradations are all related to the Total Electron Content (TEC) along the propagation path. On the other hand, the leading degradation due to irregularities is a phenomenon commonly called scintillation. These will be the main subject matter of this chapter.