Abstract:Discrete gauge symmetries were first introduced in the context of 4D particle physics as remnants of spontaneously broken gauge symmetries by Krauss and Wilczek. Following their pioneering work, we study the hidden discrete gauge symmetries in the Standard Model at the renormalizable level. There exists a discrete gauge symmetry which effectively acts as the Baryon number up to the Delta B=3 mod 3 level. This is consistent with the prediction from non-perturbative processes corrections in the SM, such as electroweak instanton and sphaleron processes. Whether there exists such symmetry will put a strong hint to new physics beyond the SM. Discrete gauge symmetries can also be used as a powerful model building tool to solve various problem in the SM as well as the MSSM, such as R-parity, mu-term, D-term splitting in SUSY flavor, and stabilizing the QCD axion solutions, etc.