This volume is dedicated to a very special person, Professor Gad Hetsroni (1934-2015). His towering figure was familiar to researchers in heat transfer and multiphase flow all over the world. He was the founding Editor of the International Journal of Multiphase Flow and the person who defined and promoted the discipline around the journal. The unique community formed in this field during his lifetime gathers every three years for a major conference, the International Conference on Multiphase Flow, that most recent was held in May 2016 in Florence, Italy. This was the first time ever Gad did not attend ICMF. Friends and colleagues from many countries came to Florence to present their personal tributes and scientific papers honoring Gad. Reviewed and edited tributes and scientific papers dedicated to Gad from these memorial sessions comprise the core content of this memorial volume; certain persons who could not participate in the ICMF made later contributions.