Vanadium-magnesium-oxides are among the most selective and active catalysts for the oxidative dehydrogenation of alkane. The selectivity for dehydrogenation depends strongly on the alkane, the Mg vanadate phase, the presence of modifiers, and to a lesser extent, the method of preparation. The results of studies on these variables are reviewed and discussed with respect to the current understanding of the nature of the active site and requirements for selective dehydrogenation.