When exposed to low temperatures, organic plastics undergo changes which may be classified broadly as reversible and irreversible. Reversible effects usually include dimensional changes due to thermal contraction and loss of moisture, increased modulus of elasticity, increased yield and ultimate strengths, decreased ductility, and—most frequently, though not in every case—decreased resistance to impact. Irreversible effects may include dimensional changes due to change of state, physical failure due to lack of sufficient ductility to respond to dimensional changes, crystallization, and freezing of plasticizers or absorbed water. In either classification, extent and rapidity of every effect varies greatly, depending upon specific environmental conditions and the particular material involved. Data are presented to illustrate these various effects. Significance from a practical point of view is shown by typical examples. Also, the importance of correct design of parts is emphasized. The paucity of data at low temperatures for such properties as fatigue strength, stress endurance, and electrical constants is pointed out.