Possible types of polysynthetic twins in micas are theoretically derived and are classified by assuming their twin operations as rotations around an axis perpendicular to the mica sheet. For each of these types of twins, the features of its x-ray diffraction pattern and their experimental evidences are presented, and the method of discriminating polysynthetic twins from polytypes are discussed. The relation between the symmetry of the mica sheet and the selective occurrence of certain types of twins is also suggested.