This Special Issue, titled “Transient Kinetics Seminar”, presents an outstanding series of papers united by the common theme: “Transient methods in heterogeneous catalysis – From scientific basics to industrial applications”. These papers were contributed (by invitation) by the participants of a virtual event with the same name that took place on 26–28 April, 2021. At that advanced stage of the COVID-19 pandemic, professional communities and distant friends had become familiar with participating in virtual gatherings that spanned many countries around the globe. Yet, those gatherings came with their own organizational challenges, and many of us felt unfulfilled with highly scripted and rarely interactive meetings online. It was from the shared needs of the research community in Heterogeneous Catalysis and Chemical Kinetics to exchange ideas, support each other, and appreciate each other’s creativity that this event has emerged. The TKS online symposium, technically hosted at the University of Oslo in Norway, thus continued an emerging tradition of lively scientific discussions on various aspects of transient kinetics that have taken place in recent years (NAM-2017, MACKIE-2018, NAM-2019). The event was co-organized by the Guest Editorial Team of this special issue with technical support provided by the IT section and Mr. Vlad Shostak at the Department of Chemistry.