This chapter presents the state of the art of experimental research conducted under outdoor conditions to evaluate soiling effects on PV systems and characterize their physicochemical properties. Initially, the methodology employed in the Systematic Literature Review (SLR) is described, encompassing from planning to the extraction and synthesis of primary studies. SLR results are organized into five thematic axes: PV technologies used, methodologies adopted, cleaning methods, effects of soiling, and physicochemical characteristics. The review shows that soiling affects multiple electrical parameters, especially short-circuit current and power, and highlights the importance of natural self-cleaning mechanisms—precipitation and wind—whose effectiveness depends on the frequency and intensity of the events. Regarding physicochemical characteristics, great variability is observed in the elemental and mineralogical composition of the deposits, influenced by geographical conditions, extreme events, and exposure time. Hence, by synthesizing trends, gaps, and challenges present in recent literature, this chapter provides a comprehensive overview of the topic and offers future research directions.