The martensitic tetragonal to monoclinic (t-->m) transformation in zirconia plays an important role in determining the mechanical properties and low temperature degradation of this ceramic. The analysis of X-ray data using the Warren-Averbach procedure can supply reliable information about crystallite size and microstrain under various conditions. The X-ray line profile analysis results can shed light on the martensitic transformation and the microstructural evolution of the t and m phases. The presence of m plates causing the formation of partially transformed t crystals reduces the mean size of the t crystals. The ball milling process also affects the mean size of the produced monoclinic crystals. The microstrain of the t and m phases can also be influenced by the ball milling and also by matrix constraints. Zirconia powders doped with low contents of CeO2 exhibit an isothermal type t-->m transformation during thermal annealing which affects the subsequent athermal transformation upon cooling and thus a decrease in the mean size of the m crystals is produced.