Rainfall variability from one year to another, i.e., on an interannual scale, is significant in the La Plata Basin. The most prominent interannual signal in the basin is associated with El Niño-Southern Oscillation (ENSO). This coupled atmospheric-oceanic mode influences monthly and seasonal accumulated rainfall values, as well as the occurrence of extreme events such as wet and dry spells, which can lead to floods and droughts, respectively. These precipitation variations can drastically impact natural ecosystems and socioeconomic activities in the basin. Many technicians, researchers, and decision-makers working in climate-influenced sectors are familiar with the ENSO phenomenon. However, understanding of its underlying causes and the many ways in which this large-scale phenomenon impacts various regions remains limited. This chapter provides a simple description of the fundamentals of interannual climate variability affecting southern South America, particularly the LPB, including the influence of ENSO and other large-scale climate patterns (e.g., the Southern Annular Mode (SAM), the Indian Ocean Dipole) on precipitation and temperature. It also includes a discussion on the interannual variability of local land surface conditions, such as soil moisture, and its influence on regional climate variability.