Plant growth regulators, endogenous and exogenously applied, are a group of chemical substances which have been ignored by most practicing forest managers and forest scientists. Yet, there is no lack of evidence in the scientific literature that endogenous regulatory substances of a hormonal nature exist in forest trees; that they influence or even exert a major control over growth, organ differentiation, tree form, transition from juvenile to mature growth form, flowering and determination of sex, and the way in which the tree responds to environmental triggers such as daylength, light intensity, temperature, and water stress. The major thrust of our discussion will be to introduce the reader to this evidence, evidence that points toward control of most growth and differentiation processes by endogenous plant growth regulators.