There are six types of leptons grouped into three families, the electron and electron neutrino, the muon and muon neutrino, and the tau and tau neutrino.

The masses of the electron, muon, and tau are 0.511 MeV, 105.7 MeV, and 1777 MeV respectively. The masses of the neutrinos are a matter of current investigation. If the neutrinos have mass, then many possibilities for interesting physics exist such as neutrino oscillations. The upper limits on the measured masses of the electron, muon, and tau neutrinos are 0.000005 MeV, 0.27 MeV, and 31 MeV respectively.

Leptons do not interact with the strong force but interact with the electromagnetic, weak, and gravitational forces. Neutrinos do not have charge and may not have mass so they interact only with the weak force. Therefore, neutrinos have very little to do with the outside world. A neutrino can pass completely through the Earth without interacting. Neutrinos can be detected however. The reaction that is used to detect them is inverse beta decay where a neutrino and an electron are absorbed, and a nucleus with different Z is created. In one neutrino detector, Cl atoms are converted to Ar atoms that are easily separated.