Magnesium components are increasingly being considered for use in vehicle structures due to the potential for weight reduction, fuel economy improvement, and emission reduction. Apart from castings, magnesium sheet components can open an entirely new opportunity for mass reduction. Magnesium’s poor ductility at room temperature, however, requires sheet forming to be carried out at elevated temperatures. The forming limits of magnesium alloy AZ31B-O were measured with both in-plane (Marciniak) and out-of-plane (limiting dome height) test methods at 300 °C. Forming limits of aluminum alloys 5182-O and 5754-O were also measured at room temperature and compared with published forming limit diagram data to validate the test procedures. Differences between the in-plane and out-of-plane test methods are discussed along with a description of failure modes and experimental challenges in obtaining strain localization and fracture in the appropriate locations. The plane strain forming limit (FLD_o) of AZ31B at 300 °C was on the order of 67% strain, which agrees well with published data.