Scandium trifluoride maintains a cubic ReO3 type structure down to at least 10 K, although the pressure at which its cubic to rhombohedral phase transition occurs drops from >0.5 GPa at similar to 300 K to 0.1-0.2 GPa at 50 K. At low temperatures it shows strong negative thermal expansion (NTE) (60-110 K, alpha(1) approximate to - 14 ppm K-1). On heating, its coefficient of thermal expansion (CTE) smoothly increases, leading to a room temperature CTE that is similar to that of ZrW2O8 and positive thermal expansion above similar to 1100 K. While the cubic ReO3 structure type is often used as a simple illustration of how negative thermal expansion can arise from the thermally induced rocking of rigid structural units, ScF3 is the first material with this structure to provide a clear experimental illustration of this mechanism for NTE.