New results on grain refinement of aluminium are presented by using the efficiency concept, which is a powerful tool to compare the nucleation efficiency of different grain refiners. This involves the calculation of nucleation sites per unit volume, which gives much more information about nucleation than intercept grain size. The different methods of measuring efficiency are discussed. Optifine, a high-efficiency grain refiner, has been on the market for almost 10 years. When Optifine is implemented at cast houses, the grain refiner additions can be minimised, using the growth restriction effect that titanium in liquid solution imposes on the first nucleated aluminium crystals. The efficiency concept is utilised in our grain refinement test method to accurately calibrate the performance of each batch of Optifine. Decreased grain refiner additions are beneficial for many reasons, e.g. lower cost, fewer impurities leading to defects, enhanced surface quality, and less blocking of filters.