Models and theories of the microstructure evolution in a directional solidification (DS) process will be firstly addressed. Discrepancies between theories and experiments will be presented. For the thin film sample growth in a temperature gradient stage, the geometrical constraint makes it difficult to compare the experimental primary spacing with theoretical predictions. While for a bulk sample growth, fluid flow always exists in the solidification process despite that the growth interface rejects a lighter or heavier solute. Based on these analyses, the appropriate techniques to conduct DS experiments are proposed that can be used to test models. One is the solidification in a capillary sample where a single cell/dendrite grows. While for array growth in 3 dimensions, long-duration microgravity experiments will be the only viable way to obtain meaningful results.