Failure mechanisms were studied in wrought AZ31B magnesium alloy under different strain paths and various temperatures. Optical micrographs were used to observe the formation of shear bands and regions of high twin density in samples strained under uniaxial, biaxial and plane strain conditions. Interrupted testing at 4% effective strain increments until failure was used to observe the evolution of the microstructure. Results showed that shear bands with a high percentage of twinned grains appeared early in the samples strained under biaxial or plane strain tension. These bands are similar to the failure region in uniaxial tension specimens. A forming limit diagram for AZ31B was developed from the strain data, showing that plane strain and biaxial tension had very similar limit strains, in contrast to materials like steel or aluminum alloys which typically have greater ductility in biaxial tension compared to plane strain tension. When the experiments are repeated at elevated forming temperatures of 75 C and 125 C there is no observable shear band formation. The forming limit diagrams for each temperature were created and showed a marked increase in biaxial tension formability, compared to plane strain tension. Optical microscopy showed no sign of any compression twins and very few tensile twins in samples strained in biaxial or plane strain tension, up to 12% effective strain. The lack of compression twins at these strain levels shows that the effect of temperature on critical resolved shear stress for < c&plus;a > slip is greater than previously expected.