Technical Abstract: Reducing water content and temperature increases the shelf life of orthodox seeds. A limit to these beneficial effects have been reported and debated over the last two decades, and guidelines for optimum seed storage remain unresolved. The central elements of the discussion are whether there are discontinuities in the effect of water content and temperature on aging kinetics and whether the discontinuities are affected by water content-temperature interactions. Biophysical measurements demonstrate discontinuous behavior of water properties within cells and that the water contents at which the discontinuities are observed increase with decreasing temperature. These effects have been reported for water sorption characteristics, glassy behavior and thermal properties, which have all been correlated with either the nature or kinetics of aging reactions. Numerous laboratories have also demonstrated discontinuous behavior in the effect of water content on seed aging kinetics and the existence of critical water contents at which survival time does not increase with further drying. There is increasing empirical evidence demonstrating that these critical water contents increase with decreasing temperature. The existence of critical water contents and temperature-water content interactions has profound implications to the maximum shelf life that can be achieved during storage. In fact, it can be demonstrated in a “thought-experiment” that if the critical water content for seed aging increases with decreasing temperature, then a critical temperature also exists and that cooling below this value will not improve longevity. Biophysical measurements on glass fragility and long term data on cryogenic storage of seeds support the possibility of a critical temperature for seed aging. A limited beneficial effect of both drying and cooling on seed longevity means that seed life spans are finite. The water content-temperature combination that provides maximum longevity for the lowest energy input and the duration that seeds can remain viable is a function of water the content-temperature interaction. That interaction remains poorly understood.