Polyamines (PAs), including putrescine (Put, a diamine), spermidine (Spd, a triamine), and spermine (Spm, a tetramine), are low-molecular-weight polycations and aliphatic nitrogen-containing substances. They play important roles in the overall life cycle of plants, from seed germination to fruit ripening, abscission, and senescence. There has been a growing interest in the study of PAs involved in plant stresses, including drought, hypoxia, high temperature, low temperature, salinity, and metal toxicity. The main role of PAs in plant stress is to counteract the damage caused by reactive oxygen species (ROS) and to prevent free radical damage or oxidative stress while also modulating ion channels to protect the morphology and integrity of cell membranes, nucleic acids, and proteins. PAs interact with hormone pathways (ethylene, jasmonate, auxin, gibberellins, cytokinins, abscisic acid [ABA], salicylic acid, and brassinosteroids) and other signaling molecules (Ca2+, NO, H2O2, and gamma-aminobutyric acid) to help plants cope with adverse environments.