The mobility of ions drifting in polar gases is explored both theoretically and experimentally. New experimental results are presented for (i) NO+ ions drifting in H2O (the reduced zero-field mobility K-0((0)) is 0.66+/-0.07 cm(2)V(-1)s(-1)), (ii) H3O+(H2O)(3) ions drifting in H2O (K-0((0))=0.43+/-0.06 cm(2)V(-1)s(-1)), and (iii) NO+(CH3COCH3)(n) ions (n=2,3) drifting in CH3COCH3 (K-0((0))=0.041+/-0.010 cm(2)V(-1)s(-1) for n=2 and K-0((0))=0.050+/-0.015 cm(2)V(-1)s(-1) for n=3). A number of theoretical models for ion mobilities in polar gases are described. The models are compared with the available experimental data and a reasonable agreement is obtained. For larger cluster ions the measured mobilities are considerably smaller than the calculated values. Some possible reasons for the discrepancies are discussed, including momentum transfer outside the capture cross section, dipole-dipole interactions, ligand exchange, inelastic collisions, and the validity of Blanc’s law.