Reactive oxygen species play a physiological role in cell signaling and also a pathological role in diseases, when antioxidant defenses are overwhelmed causing oxidative stress. However, in this review we will focus on reductive stress that may be defined as a pathophysiological situation in which the cell becomes more reduced than in the normal, resting state. This may occur in hypoxia and also in several diseases in which a small but persistent generation of oxidants results in a hormetic overexpression of antioxidant enzymes that leads to a reduction in cell compartments. This is the case of Alzheimer’s disease. Individuals at high risk of Alzheimer’s (because they carry the ApoE4 allele) suffer reductive stress long before the onset of the disease and even before the occurrence of mild cognitive impairment. Reductive stress can also be found in animal models of Alzheimer’s disease (APP/PS1 transgenic mice), when their redox state is determined at a young age, i.e. before the onset of the disease. Later in their lives they develop oxidative stress. The importance of understanding the occurrence of reductive stress before any signs or symptoms of Alzheimer’s has theoretical and also practical importance as it may be a very early marker of the disease.