This review of the mechanical properties of paintings places the conservation literature in the broader context of the industrial paint and polymer literature. The layers in paintings that create and carry most of the tension (varnish, paint, ground, and size) are all amorphous or semi-amorphous polymers. Short times (such as shock and vibration), low temperature, low humidity, high pigmentation, and aging increase both the stiffness and the likelihood of cracking of these layers, and the unifying concept is the glassy/rubbery transition. The stiffness data on oil paints, glue, acrylics, and resins is charted in terms of this transition. Low temperature and low humidity also shrink materials, and this data is summarized. All the tension models - experimental, simple equation, and computer - are consistent in their predictions. Extreme but plausible conditions of shock, temperature, and humidity may exceed the strengths of some new materials, but unfortunately the tolerance of old and weak paintings to moderate conditions remains unclear.