With the advent of multi collector-inductively coupled plasma mass spectrometry, stable isotopic variations of non-traditional metal elements have provided important constraints on the sources of geologic materials. This review introduces the principles of magnesium isotopic fractionation and analytical methods. Recent case studies are also reviewed for the use of magnesium isotope signatures to decipher the source materials of I-, S-, and A-type granitoids in western North America, Australia, and China.