Molecular rotary motors belong to a growing family of functional molecular devices capable of transforming chemical, electric, or light energy into mechanical work, offering unprecedented means for controlling motion at the nanoscale and opening way for design of interactive, 'smart' molecular systems and materials for countless applications. Among other molecular systems of such kind, often referred to as molecular machines, 2,5,15-17 light-driven rotary motors characterize with continues unidirectional rotary motion under a flux of photons of energy matching the electronic absorption spectrum of the motor. In the course of this motion, a part of molecular system considered static, the 'stator', constitutes a reference framework for the other, mobile part, the 'rotor', and the unidirectionality of the rotation stems from the inherent system chirality. The historically first and the nowadays most populated group of molecular motors are overcrowded alkenes, in which the light-triggered rotation occurs about a double C=C bond. It is important to note that, amid the overcrowded alkenes, three generations of motors can be distinguished: (i) the first-generation, in which stationary and rotating parts of the molecule are identical, both possessing same-oriented stereogenic centres, (ii) the second-generation, in which two halves of the motor are different and both, or only one of them, possess a stereogenic centre, and (iii) the third-generation, in which, contrary to the previous two, the motor consists of one pseudo-chiral stator and two rotors, bonded on its opposite sites.