Reaction wood originates from cambial activity which will adjust cell division activity, cell wall structure and properties, so that the resulting growth event will be the appropriate response to endogenous and environmental stimuli. When addressing the question of the induction of reaction wood formation, the physical parameters inducing reaction wood are first presented leading to discuss the importance of gravisensing versus proprioception (sensing of the local curvature of the stem) in this process. Molecular candidates for the perception of cellular deformation that is hypothesized to occur in a gravistimulated stem are located at CPMCW (cytoskeleton-plasma membrane-cell wall) continuum. These candidates would mediate intracellular signalling. Insights from global approaches (e.g. transcriptome and proteome analyses) performed on tilted trees suggest calcium, reactive oxygen species and phosphatidy linositol signalling in the gravitropism sensing network. It has been unambiguously shown that several of the aux/IAA gene family mediators of auxin signal transduction pathway change on induction of tension wood formation. Gibberellins and ethylene seem also to be involved in reaction wood formation. The role of the different plant hormones in upstream primary response to reaction wood sensing or alternatively in the transmission of the signal from the perception to the reaction wood forming cells is discussed.