In this chapter, both, the variability and trends of climatological variables and of indirect factors—the cosmogeophysical and the terrestrial ones—will be discussed. All the sections are developed keeping in mind the potential influence of such observables on drought occurrence. The first part is focused on the climate system, as seen from our planet, dividing the text into two sections: the first section deals with meteorological drought, and the second with hydrological drought. The second part, in which more critical issues are discussed, deals with the Sun–Earth influence, galactic cosmic rays, and aerosols.