The term operando was coined at the beginning of this century to gather the growing efforts devoted to establish structure-activity relationships by simultaneously characterizing a catalyst performance and the relevant surface chemistry during genuine catalytic operation. This approach is now widespread and consolidated; it has become an increasingly complex but efficient junction where spectroscopy, materials science, catalysis and engineering meet. While for some characterization techniques kinetically relevant reactor cells with good resolution are recently developing, the knowledge gained with magnetic resonance and X-ray and vibrational spectroscopy studies is already huge and the scope of operando methodology with these techniques is recently expanding from studies with small amounts of powdered solids to more industrially relevant catalytic systems. Engineering catalysis implies larger physical domains, and thus all sort of gradients. Space- and time- resolved multi-technique characterization of both the solid and fluid phases involved in heterogeneous catalytic reactions (including temperature data) is key to map processes from different perspectives, which allows taking into account existing heterogeneities at different scales and facing up- and down-scaling for applications ranging from microstructured reactors to industrial-like macroreactors (operating with shaped catalytic bodies and/or in integral regime). This work reviews how operando methodology is evolving toward engineered reaction systems.