Food powders should appear homogeneous and keep free-floating properties during storage and chemical changes should be minimized. The shelf-life of food powders as influenced by both physical changes and chemical reactions is controlled by the availability of water. Crystallization together with a non-enzymatic browning and oxidative deterioration is prevented in food powders in glassy states, and water activity and temperature together determine critical storage conditions. For milk powders, lipid oxidation determines shelf-life at ambient temperature, and intermediate water activity is recommended for storage. In contrast, non-enzymatic browning dominates at higher temperatures, where lower water activity minimizes browning reaction. The construction and use of temperature/water activity phase diagrams to define storage conditions for milk powders and other lipid or protein-rich food powders are strongly encouraged.