Hormones are key regulators of plant growth and development. Genetic and biochemical studies have identified major factors that mediate ethylene biosynthesis and signal transduction. Substantial progress in the elucidation of the ethylene signal transduction pathway has been made, mainly by research on Arabidopsis thaliana. Research on ethylene biosynthesis and its regulation provided new insights, particularly on the posttranslational regulation of ethylene synthesis and the feedback from ethylene signal transduction. The identification of new components in the ethylene-response pathway and the elucidation of their mode of action provide a framework for understanding not only how plants sense and respond to this hormone but also how the signal is integrated with other inputs, ultimately determining the plant phenotype.