This book focuses on recent advances in our understanding of the signal transduction pathway of ethylene, its interaction with other hormones and its roles in biological processes. It discusses at which point plants could have acquired ethylene signaling from an evolutionary perspective. Ethylene was the first gaseous hormone to be identified and triggers various responses in higher plants. Our grasp of ethylene signaling has rapidly expanded over the past two decades, due in part to the isolation of the components involved in the signal transduction pathway. The book offers a helpful guide for plant scientists and graduate students in related areas.