Ethylene is a gaseous plant hormone that has profound eects on many aspects of plant growth and development. e role of changing levels of ethylene in modulating fruit ripening, seed germination, hypocotyl and root elongation, abscission, and responses to biotic and abiotic stresses has been well described (Abeles et al. 1992; Kendrick and Chang 2008). e best studied ethylene-regulated processes are hypocotyl elongation, particularly in dark-grown seedlings, and fruit ripening (as reviewed in Klee 2004; Giovannoni 2007; Kendrick and Chang 2008). Altered ethylene response in these two growth processes are the phenotypes used for isolation of mutants in ethylene signaling and synthesis, predominantly in Arabidopsis thaliana and Lycopersicum esculentum (tomato) (Klee 2004; Giovannoni 2007; Kendrick and Chang 2008). In recent years, the eect of ethylene on root growth and development has received substantially more attention, with identication of both inhibitory eects of ethylene on root elongation, gravitropism, and lateral root development and stimulatory eects of ethylene on root hair initiation. ese recent studies using the plethora of ethylene signaling and synthesis mutants have identied mechanisms by which ethylene regulates root growth and development and have provided strong evidence for ethylene exerting its eect on root growth through interactions with auxin. is chapter will introduce the basics of ethylene signaling and how ethylene aects root growth and development, followed by sections that detail the mechanisms by which ethylene interacts with auxin signaling, transport, and synthesis to drive ethylene-dependent root growth and development.