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Synopsis There are five classes of compounds in the premier division of endogenous plant growth regulators. These are auxins, gibberellins (GAs), cytokinins (CKs) abscisic acid (ABA) and ethylene. Broadly speaking, the auxins and GAs have been classified as regulators of cell elongation, the CKs as regulators of cell division. ABA as a general inhibiting influence and ethylene as a volatile with a finger in numerous "developmental pies". Descriptions such as these are clearly simplistic and represent vain attempts to dispense plant growth regulators into neat compartments on the basis of their effects on plant tissue. This common practice is misguided, not only because it implies that a plant growth regular has a similar impact on the behaviour of any plant cell, but also and more significantly because it assumes that the application of a plant growth regulator must mimic its effect in situ. The balance of evidence indicates that neither of these assumptions is correct. In addition, it is native to believe that the five groups of plant growth regulators described above may be the only molecules that can influence plant growth and development: an increasing number of other compounds such as the jasmonate, polyamines, brassinosteroids and salicylate are being reported to have potent effects on plant tissues. No book of this size can realistically lead the reader step by step from the discovery of the various plant growth regulators to the forefront of modern plant growth regulator research. Therefore, a certain amount of background knowledge has had to be assumed. Furthermore, it has not been our intention to produce an encyclopedic work describing every developmental event where plant growth regulators have been implicated. Rather we have preferred to concentrate on a critical appraisal of plant growth regulators in the regulation of representative processes. In keeping with a volume of this size, we have assumed that the reader has a sound knowledge of plant physiology and biochemistry. However, wherever possible, we have highlighted useful reviews, which provide background information, along with recent publications that have contributed significantly to the literature.