High-valent oxo-complexes have recently emerged as powerful catalysts for the activation of X–H (X = Si, B, P and H) bonds and for the reduction of several functional groups. This new reactivity represents a complete reversal from the traditional role of these complexes as oxidation catalysts and opened a new research area for high-valent oxo-complexes. This tutorial review highlights the work developed using high-valent oxo-molybdenum and oxo-rhenium complexes as excellent catalysts for X–H (X = Si, B, P and H) bond activation and for organic reductions.