Cells are continuously exposed to different types of stress, either by products of their own metabolism or by environmental changes in reactive oxygen species (ROS) levels, pH, osmotic pressure and temperature. Extensive literature has demonstrated that PAs are associated with the response and protection to different types of stress which might involve multiple PA properties. It is usually observed that PA intracellular concentrations change during exposure to stress. Moreover, modification of PA concentrations either by exogenous addition or chemical/ genetic methods can influence the sensitivity of cells to stress (reviewed in Refs. [5] and [6]). An example of this is the accumulation of Put during osmotic stress in rat hepatoma cell cultures and plants. In rats, this effect is due to an increase in ODC activity caused by a decrease in the stability of the regulatory enzyme of ODC, AZ [see The regulation of polyamine synthesis through antizyme (AZ) levels] [178]. In plants, when osmotic stress occurs, Put accumulates to toxic levels due to an increase in ADC activity [179] and [180], resulting in chlorophyll loss and senescence [181]. Treatment with Spm protects cells from these toxic effects [181] by inhibiting ADC proenzyme posttranslational activation and therefore ADC activity [182]. This modification mediated by Spm suggests that ADC is regulated at the posttranslational level during osmotic stress [179] and [181].