Although rotational intramolecular motion is well known 31 also in crystalline solids, stimulidriven unidirectional rotation in a molecular motors represents the ability to control the frequency and directionality by molecular design. Using stepwise successive photoisomerisation and thermal relaxation (Fig. ) these molecules a rotor can unidirectionally rotate around the axle represented as the double bond of an overcrowded alkene. A broad variety of artificial light driven molecular motors with different structures, properties and functionalities were established by changing the molecular design. Portions of this figure are reprinted from Ref. .