Unintentional impurities can play a significant role in reducing the efficiency of crystalline silicon solar cells. With the advent of low-cost solar-grade silicon feedstocks, this is likely to remain the case well into the future. The purpose of this paper is to review the most important impurities in directionally-solidified ingot-grown multicrystalline silicon, their chemical states, where they come from, and the most commonly used techniques to detect them.