Rolled sheets of AZ31 magnesium alloys were tensile tested at room temperature. Twinning behavior was observed at various strain levels up to a pint of fracture, using an optical microscope, an orientation imaging microscope and an acoustic emission system. Twinned area was increased in two separate stages with strain. Initial rapid increase was observed near the yield point, due to the formation of wide lenticular twins. This was followed by another rapid increase close to the failure point, due to the formation of narrow banded twins. Acoustic emission (AE) signals also exhibited two separate stages with strain, which enabled us to relate each twin type with FFTed AE signals. The each type of twin will be discussed in light of its role on deformation and fracture mechanisms.