Operando spectroscopy is a methodology that combines the spectroscopic characterization of a catalytic material during reaction with the simultaneous measurement of the actual performance of the catalyst, e. g. conversion and selectivity. The simultaneous quantitative assessment of the catalytic performance differentiates operando spectroscopy from in situ spectroscopy. Unfortunately, confusion regarding this difference has led to the use of the grammatically incorrect term "in operando" in recent literature. In situ means "in place", whereas "operando" means "working" or "operating", i.e. the catalyst at work, so it is incorrect to state "in working", which is what in operando would imply. Operando spectroscopy was formally introduced into the catalysis community around 2002, with the idea that a new term was required to emphasize the importance of simultaneously combining fundamental spectroscopic measurements with quantification of catalytic performance, on the same sample, under realistic reaction conditions. It is important to point out that operando studies are focused on intrinsic kinetics rather than reactions that are limited by transport limitations etc. The potential impact of operando spectroscopy on catalysis science lies in its ability to significantly assist in the establishment of fundamental molecular structure-activity/selectivity relationships for catalytic systems. It is critical that the design of the spectroscopic reaction cell, the catalytic reactor, also allows for the generation of catalytic performance data that is analogous to those achieved with conventional catalytic reactors. Every three years, beginning with a conference held in Lunteren, The Netherlands in 2003 there has been an International Congress on Operando Spectroscopy (Toledo, Spain in 2006; Rostock, Germany in 2009; and Brookhaven, USA in 2012). This conference series has become the premier venue for researchers from around the world to present their latest work, debate, and discuss the present and future of operando catalyst characterization. (C) 2013 Published by Elsevier B.V.