A diradical in organic chemistry is a molecular species with two electrons occupying two degenerate molecular orbitals (MO). They are known by their higher reactivities and shorter lifetimes. In a broader definition diradicals are even-electron molecules that have one fewer bond than the number permitted by the standard rules of valence. If the electrons are of antiparallel spin, the molecule is said to be in a singlet state. Alternatively, the electrons can be of parallel spin, which constitutes being in the triplet state. The related radical has just one free electron. The phrases singlet and triplet are derived from the multiplicity of states of diradicals in electron spin resonance: a singlet diradical has one state (S=0, Ms=2*0+1=1, ms=0) and exhibits no signal in EPR and a triplet diradical has 3 states (S=1, Ms=2*1+1=3, ms=-1;0;1) and shows in EPR 2 peaks (if no hyperfine splitting). The triplet state has total spin quantum number S = 1 and is paramagnetic. The singlet state has S = 0 and is diamagnetic. The degeneracy of each state can be found with Hund's rule of maximum multiplicity: 2S + 1. In molecules the free electrons can reside on one atom or on different atoms. A molecule can have a singlet state or triplet state with different energy and both states can inter-convert by a process called intersystem crossing. Phosphorescence is based on this principle. Discrete molecules with a diradical nature are singlet oxygen and triplet oxygen. Other important diradicals are carbenes and nitrenes. Lesser known diradicals are nitrenium ions, carbon chains and organic so-called non-Kekulé molecules in which the electrons reside on different carbon atoms.