Аннотация:Summary In this chapter, we discuss the nature of nanoparticle catalysts and the mechanistic pathways of catalytic transformations. Evolution of catalytic centers involves a number of dynamic processes taking place in reaction mixtures during catalytic reactions. We briefly discuss applications of electron microscopy, mass spectrometry, DFT calculations, mercury test, and other techniques to reveal true active species in the catalytic reactions. It is generally admitted that nanoparticle catalytic systems are dynamic and often behave in a cocktail-type manner.