As the Standard model on particle physics predicts lepton number and flavor conservation in any process except neutrino mixing, any observation on charged lepton flavor violation (cLFV) is a strong signature of the existence of theories beyond Standard model. This talk will cover an overview of theoretical and experimental aspects of cLFV, both on tau and muon leptons. The status of current muon conversion experiments in J-PARC and Fermilab, and requirements of future cLFV experiments will be emphasized.