This book constitutes the thoroughly refereedpost-conference proceedings of the 9th International Conference on QuantumInteraction, QI 2015, held in Filzbach, Switzerland, in July 2015.The 20 papers together with 2 invited keynotes presentedin this book were carefully selected from 27 submissions. Quantum Interactionhas developed into an emerging interdisciplinary area of science combiningresearch topics in mathematics, physics, psychology, economics, cognitivescience, and computer science.