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Xanthine oxidoreductase (XOR) is among the most-intensively studied enzymes known to participate in the consumption of oxygen in eukaryotic cells. However, it attracted the attention of researchers due its participation in free radical production in vivo, mainly through the production of superoxide radicals. In plants, XOR is a key enzyme involved in purine degradation where it catalyzes the oxidation of hypoxanthine to xanthine and of xanthine to uric acid. Both reactions are accompanied by electron transfer to either NAD+ with simultaneous formation of NADH or to molecular oxygen, which results in formation of superoxide radicals. Characterization of plant XOR mutants and isolated XOR proteins from various plant species provided evidence that the enzyme plays significant role in plant growth, leaf senescence, fruit size, synthesis of nitrogen storage compounds, and plant–pathogen interactions. Moreover, the ability of XOR to carry out redox reactions as NADH oxidase and to produce reactive oxygen species and nitric oxide, together with a possible complementary role in abscisic acid synthesis have raised further attention on the importance of this enzyme. Based on these established and proposed functions, XOR is discussed as regulator of different processes of interest in plant biology and agriculture.