In this chapter, the synthesis and structural characterization of organoselenium (RxSez+y; z=1–3) cations are described. Structurally characterized organoselenium cations are the main focus of the discussion. The reactivity of the organoselenium cations generated in situ in synthetic transformations, particularly in the selective functionalization of alkenes is discussed. Their use as sensors and antioxidants is briefly presented. The brief review presented in the chapter provides insights into the isolation of notoriously reactive species and their potential uses as future smart materials, catalysts for greener and enantioselective transformations and as antioxidants.