Wood formation is a fundamental biological process of significant economic andcommercial interest. During wood formation, most glucose from the carbohydratemetabolism is channeled to cellulose in the secondary cell walls. The cellulose microfibrils associate with hemicellulose, proteins, and lignin to form the strong and flexiblebiocomposite known as wood. As the main wood component, cellulose is essential forthe survival of trees and for their exploitation by man.In spite of this, the molecular details of cellulose biosynthesis have remained obscure in all plants. In particular, the toughness of wood cells makes it hard to isolateactive enzymes and study cellulose synthesis in trees. Functional genomics providespowerful new tools to study complex metabolic processes. In this way, 18 CesA geneshave been recently identified in the genome sequence of Populus trichocarpa.Expression profiling during wood formation has shown that four of these genesare specifically upregulated during xylogenesis and/or tension wood formation. Othergenes that follow the same expression pattern as the wood-related CesA genes encodethe putative Korrigan ortholog PttCel9A and a novel microtubule associated proteinPttMAP20. Cell suspension cultures of hybrid aspen with elevated expression of thesecondary cell wall specific PttCesA genes have been used for efficient in vitro synthesisof cellulose, which will facilitate future studies of this challenging process in trees.