Walnut trees (Juglans sp.) produce high quality timber, valuable fruits and secondary products (oil, alcohol, dyes). However, vegetative and sexual propagation of these trees is difficult so that demand remains superior to supply. Different in vitro systems were developed to produce plants or study developmental processes: (i) somatic embryogenesis, (ii) adventitious root formation from cotyledon fragments and (iii) micropropagation. The genetic origin of the material used is controlled (J. regia; J. nigra x J. regia). It was obtained from immature zygotic embryos, mature embryo axis or axillary buds from selected adult trees. Improvement of medium and cultural conditions used for micropropagation resulted in multiplication rates of 1.5-2.5/3 weeks and 25 to 90% rooting efficiencies. Subsequent radial growth of the stem can also be obtained in vitro under specific culture conditions. Cotyledon fragments were used to determine the origin of adventitious roots and study their formation through enzyme markers (Duroux et al., Biochem J, in press) and cDNA cloning. Several embryonic lines have also been stabilized. One hybrid line was used for transformation. Six transgenic lines expressing antisense chs gene showed contrasted flavonoid contents and rooting abilities (El Euch et al., Plant Mol Biol, in press). Interestingly, three J. regia lines were obtained from "early mature tree" phenotypes. Microshoots presenting putative terminal flower buds were observed after 7 subcultures. Walnut in vitro systems are complementary and easily connected one to another. They provide good opportunities to study key processes in one recalcitrant woody species: root development, cambium activity and wood formation, flower induction.