Formability is the extent to which a material can be deformed in a particular process before the onset of failure. Aluminum sheet and aluminum shapes usually fail by localized necking or by ductile fracture. Precipitation-strengthened aluminum alloys usually are formed in the naturally aged (T4) condition, or in the annealed (O) condition, but only very rarely in the peak-strength (T6) condition where both the necking and fracture limits are low. Curves can be plotted for most of the precipitation-strengthened alloys in the 2xxx and 6xxx series showing the effect of a wide range of precipitation structures on some of the forming properties.