Adventitious root formation (ARF) is an important step in the vegetative propagation of plants for horticulture, forestry, and agriculture. As a physiological process, adventitious rooting is regulated by multiple plant growth regulators that partake in a complex web of interactions. The exogenous application of synthetic and natural plant growth regulators is common practice for inducing ARF for plant propagation in industry. In this review, we revisit the established, central function of auxins in ARF, and discuss the mechanism of their activity. We also discuss the effects of two other plant growth regulator classes with established roles in adventitious rooting, ethylene and cytokinins, and summarize our present understanding of how these three growth regulator classes interact. The effects of abscisic acid, gibberellins, brassinosteroids, salicylic and jasmonic acids, and nitric oxide on ARF are also presented. Throughout, we emphasize the interactions between growth regulator classes, how these affect ARF, and the relevance of these interactions for the propagation of ornamentals. This review will provide a better understanding of adventitious rooting as an intricate physiological process, and will demonstrate the complexity of induction of ARF by exogenous hormonal treatments.