The development of in situ techniques and the introduction of operando studies have had a vast impact on research and developments in heterogeneous catalysis. Over the past 40 years, we have gone from a situation where only limited knowledge existed about the state of the active catalyst to a situation where atomic-scale insight about the catalyst and intermediates can routinely be acquired. In fact, it has recently been shown that atomic-resolved images may be obtained in situ for most catalyst systems. Operando studies have shown that catalysts may undergo dynamic structural transformations upon small changes in the reaction conditions and such transformations have a strong impact on the performance of the catalysts. The fact that, strictly speaking, the active state of a catalyst only exists during the catalysis further emphasizes the need for performing operando studies under relevant reaction conditions. The fundamental insight which can be obtained from in situ and operando studies has been important in catalyst developments since it has made more rational catalyst design strategies possible. The progress in in situ characterization and operando studies is illustrated by examples taken mainly from research on hydrodesulfurization and methanol synthesis catalysis. Some of the present major limitations and likely future developments will also be discussed. (C) 2003 Elsevier Science (USA). All rights reserved.