This chapter describes the origins of solar radiation caused by flares from the Sun and their effects on the Earth’s ionosphere and geomagnetic field. The solar effect can be both constructive and destructive for ionospheric radio communications and usually destructive for satellite communications. Constructive solar effects cause the formation of a regular (desired) Earth ionosphere with ionized regions D, E, and F, which support ionospheric communications. Distractive solar effects occur in the form of sudden ionospheric disturbances (SIDs), ionospheric storms (ISs), and polar blackouts, which may disrupt both HF and satellite communications. All of these regular and irregular phenomena are described in this chapter. Additionally, in this chapter, important ionospheric parameters, such as the number of sunspots, solar radio flux, critical region frequencies, and geomagnetic indices, are described. Furthermore, methods of HF link optimization are presented, keeping on mined ionospheric communication parameters, such as the maximum usable frequency (MUF), low usable frequency (LUF), and signal fading due to multipath ionospheric propagation. The effective and reliable optimization of ionospheric radio links at a time and for a geolocation of interest includes ionospheric data from ionospheric forecast services, which are given in this chapter. At the end, ionospheric effects on satellite communications are discussed, including satellite signal propagation delays, dispersion, scintillation, and changed wave polarization with Faraday’s rotation. Besides solar radiation, this chapter will also briefly cover less EMI-critical cosmic sources like lunar, stellar, and galactic radiation.