In the past, several runaways have occurred in Trickle-Bed Reactors (TBR) used for the hydrogenation of pyrolysis gasoline as produced in ethylene cracking installations. This phenomenon has been studied in the framework of a special program in the Netherlands, which is administered by the National Science Foundation and which aims to keep scientific groups in the former Soviet Union working in their current difficult times. To this end a working party was set up at the Boreskov Institute of Catalysis (BIC) in Novosibirsk, Siberia in Russia. The day-to-day management of the working party was conducted locally by Professor Kirillov and the authors Westerterp and Kronberg scientifically and administratively managed the project. This paper reports the results of the study that took place in the years from 1997-2000. The study has led to certain recommendations for the design and operation of TBR's for the hydrogenation of pygas, which will be elucidated and explained here. To this end the phenomenon of "particle runaway" will first be discussed, and then the major results of the study at the BIC will be described, and eventually conclusions will be reached for the running of pygas hydrogenation TBR's. Later, in depth reports will be published on all work done, but now only the most important results are discussed, without going into detail.