Polyamines (PAs) are small polycationic, biogenic amines that have profound effects on growth, development, and senescence in eukaryotic cells (Galston and Kaur-Sawhney, 1995; Cassol and Mattoo, 2003; Casero and Marton, 2007). Diamine putrescine (Put) is a major PA in plants and a precursor for triamine spermidine (Spd) and tetraamine spermine (Spm). PAs influence many biochemical and physiological processes such as cell division, cell elongation, flowering, fruit set and development, fruit ripening, and senescence (Evans and Malmberg, 1989; Galston and Kaur-Sawhney, 1990; Bouchereau et al., 1999). Many of these processes have direct implications on various aspects of postharvest biology of fruit and vegetable crops including quality, storage life, senescence, chilling and other stresses, and disease development (Valero et al., 2002a). Significant information about PA action has emerged from indirect studies employing pharmacological levels of different PAs and as well as their biosynthetic inhibitors. In recent years, reverse genetics has begun to provide direct evidence for involvement of PAs in determining fruit quality, ripening, and processing attributes (Tiburcio et al., 1997; Martin-Tanguy, 2001; Mehta et al., 2002; Mattoo et al., 2006; Srivastava et al., 2007). Molecular aspects of PA action have also begun to emerge, especially from nonplant systems (Igarashi and Kashiwagi, 2006; Wallace and Niiranen, 2007). PAs have been shown to bind to membranes, nucleic acids, and other macromolecules, and have been implicated in stabilizing chromatin conformation, regulating ion channels, scavenging free radicals, and regulating gene expression (Casero and Marton, 2007; Srivastava et al., 2007). Since maintenance of membrane stability and homeostasis is essential for many cellular, physiological, and biochemical processes, it is likely that PAs can have a pivotal role in extending postharvest shelf life of fruits and vegetables. The focus of this chapter is on recent progress made in understanding the roles of PAs in plants with special emphasis on postharvest biological processes.