Book excerpt: During the last two decades there has been considerable growth in the development of electrospray ionization mass spectrometry (ESI-MS) as a practical method in the study of reaction mechanisms. This method allows the interception and characterization of key intermediates, either as transient species or as protonated/deprotonated forms of neutral species by API-MS. The outstanding features and advantages of ESI-MS make it one of the most suitable tools for the fast screening of intermediates directly from solution, providing hitherto unavailable chemical information to organic chemists. This monograph provides an overview of the mechanisms involved in ESI-MS, the historical perspectives before looking further in-depth at specific reactions and intermediates. Written by researchers in the field, this book is an unique resource for the understanding of this cutting-edge technique.