Nickel is an essential nutrient for plants. However, the amount of Ni required for normal growth of plants is very low. Hence, with the level of Ni pollution in the environment increasing, it is essential to understand the functional roles and toxic effects of Ni in plants. We briefly review advances in relevant research over the past 20 years. Based on the available data, two new indirect pathways of Ni toxicity in plants are proposed. These are (i) interference with other essential metal ions and (ii) induction of oxidative stress. Research should focus on these mechanisms at the protein and molecular levels. Further research should also be directed at plant species that are capable of accumulating Ni at high concentration, so-called hyperaccumulators. Such species can provide model systems to study the mechanisms of Ni tolerance and can also be used for phytoremediation by removing nickel from polluted environment.