Usually, when considering Mott transitions, one treats the system as homogeneous – either with localized, or with itinerant electrons. However, such localized – itinerant crossover can occur not nesessarily homogeneously in the whole sample, but there may appear in a solid small clusters –“molecules” (dimers, trimers, heptamers etc) in which electrons already can be treated as weakly-correlated, whereas the hopping between such clusters may still be small, so that the whole material may still be insulating. In this talk I will discuss the conditions for such phenomenon (in particular, low-dimensionality and magnetic frustrations usually facilitate creation of such clusters), present several examples, and consider some special properties of such molecular clusters, such as e.g. the orbital-selective Peierls transition with partial quenching of magnetic moments.