According to one aspect, there is provided a method of selectively solidifying successive layers of build material in a 3D printing system. The method comprises obtaining data relating to a 3D object to be generated, forming a layer of build material on a build platform, applying to the formed layer a pattern of fusing agent corresponding to a portion of the layer to be solidified to form a layer of the object, applying to the formed layer a pattern of anti-caking agent on portions of the formed layer that are not to be solidified to form a layer of the object, and applying energy to the whole of the formed layer to cause portions of the layer on which fusing agent was applied to heat up, melt, coalesce, and then solidify upon cooling, and to cause portions of the formed layer on which anti-caking agent was applied to control caking of build material in those portions.