The influence of polar species on the transport of charge carriers in molecular solids is reviewed. Guest molecules with permanent dipole moments introduced into molecular crystals can act as traps localizing carriers provided certain energetic conditions are fulfilled. In this case their polar character may manifest itself in a field-dependent decrease of the trap depth. Moreover, irrespective of the positions of their energy levels, polar impurities may locally modify the polarization energy thus creating traps on neighbouring molecules. The effect of polar dopants in disordered molecular solids can be described within the model due to Bässler: the presence of polar species broadens the density-of-states function thus decreasing the effective mobility of charge carriers.