During chemical pulping, lignin has to be removed from cellulose. This process is toxic, energy consuming, and results in a low biomass utilization. For the pulp industry, it would be beneficial to process trees which have either less lignin, or a modified lignin that is easier to separate from cellulose. Trough genetic engineering, we have modified the expression of genes that play crucial roles in lignin biosynthesis in poplar, i.e. those encoding caffeic acid/5-hydroxyferulic acid-O-methyltransferase (COMT), caffeoyl-CoA-O-methyltransferase (CCoAOMT), cinnamoyl-CoA-reductase (CCR), cinnamyl alcohol dehydrogenase (CAD) and putative lignin-specific peroxidases, and studied the impact on Kraft pulping. Inhibition of the COMT activity in transgenic poplar results in a significant change in the composition of lignin with the simultaneous appearance of a novel lignin unit. Due to the more condensed lignin in these transgenic trees, the lignin is more difficult to extract during chemical pulping, as indicated by the higher kappa number. In contrast, inhibition of CAD activity in poplar results in different lignin characteristics resulting in an increased lignin extractability during chemical pulping. Down-regulation of CCoAOMT results in a reduction in the amount of lignin as well as in a lower kappa number. The analysis of transgenic poplars with a modified CCR level is underway and field trials for antisense CAD and antisense COMT plants are running. In addition, as a start to explain the heterogeneity in lignin composition observed between the different wood cell types and under different environmental conditions, the cellular and subcellular expression of both COMT and CCoAOMT have been analysed and compared by immunolocalisation. This work was carried out in the framework of the EU Research Programs AGRE-0021-C, FAIR-PL 95424, and AIR2-CT93-1661.