Wood is a complex and highly variable tissue, the formation of which is developmentally and environmentally regulated. In reaction to gravitropic stimuli, angiosperm trees differentiate tension wood, a wood with specific anatomical, chemical and mechanical features. In poplar the most significant of these features is an additional layer that forms in the secondary wall of tension wood fibres. This layer is mainly constituted of cellulose microfibrils oriented nearly parallel to the fibre axis. Tension wood formation can be induced easily and strongly by bending the stem of a tree. Located at the upper side of the bent stem, tension wood can be compared with the wood located on its lower side. Therefore tension wood represents an excellent model for studying the formation of xylem cell walls. This review summarizes results recently obtained in the field of genomics on tension wood. In addition, we present an example of how the application of functional genomics to tension wood can help decipher the molecular mechanisms responsible for cell wall characteristics such as the orientation of cellulose microfibrils.