Plant growth and development are tightly regulated by compounds produced in trace amounts in the plant. Besides the classical phytohormones, many plant metabolites have been described to affect plant development. Among these are several phenylpropanoids, although conclusive evidence for their bioactivity at physiologically relevant concentrations is only available for cinnamic acid. By inhibition of auxin efflux transport, the cis-isoform of cinnamic acid alters auxin homeostasis, resulting in auxin-related growth effects. Despite insight into its mode of action, the molecular target of cis-cinnamic acid is not yet known, and it remains to be determined whether this or other phenylpropanoids have a role to play in regulating plant growth and development under normal or stress conditions.