The presence of thermal transitions within the working range of denture base polymers may have a drastic effect on the dimensional stability of the material. The glass transition temperatures of fifteen denture base polymers have been measured and their significance discussed. The effect of thermal contraction and polymerization contraction on dimensional stability is not fully understood at the present time. An attempt has been made to explain the dimensional changes which occur in denture bases with the aid of transition temperatures and inferred thermal contraction and polymerization contraction values.