The ethylene receptors of plants are related to and originated from bacterial histidine kinases. As such they represent a system by which one can study not only how the ethylene signal is perceived and its signal transduced, but also how bacterial two-component systems have been adapted for signal transduction in a eukaryote. Much of the biochemical characterization of the ethylene receptors, including the demonstration of kinase activity, ethylene binding, and interaction with other signaling components, has relied on the ability of the receptors to be functionally expressed in transgenic yeast. This chapter describes some of the key approaches used for such work, with a special emphasis on techniques employed to analyze ethylene binding. In many cases the approaches used in transgenic yeast may also be used for studies of the receptors in the native plant.