This book addresses the redox balance in cells and organisms, which is of paramount importance in biology. In addition, a generally overseen circadian modulation is underlined. The balance involves, on the one hand, free radicals and reactive oxygen species and, on the other hand, scavengers capable of neutralizing their effects. This balance is crucial for life, since free radicals are required for some basic reactions (such as the activation of an enzyme involved in DNA synthesis) and since their excess leads to various toxic effects, most notably cancer. Different scavengers can be distributed at various times of the day, their amount possibly peaking at particular times. In fact, circadian rhythms may affect amounts of scavengers, amounts of foreseeable free radicals or the effects the latter can exert on a system. Free radicals are produced by some chains of normal metabolism. They are also produced by a variety of environmental stresses, UV light, chemicals, drugs, etc.