Maturation in conifers includes several distinct and persistent changes in the growth habits of apical meristems. Despite many studies on maturation in conifers, there are still many aspects of the process that have not been elucidated. For example, it is not known why maturation of cotyledon-derived tissue culture plantlets is rapid, whereas the natural maturation process is gradual. Also, it is not known whether rejuvenation occurs as a result of mature cells reverting to the juvenile state, or whether rejuvenation results from selective multiplication of cells that have never matured. In this paper, I review the primary causes of maturation in woody plants, with emphasis on gene expression and its role in cell determination during the maturation process. Recent experiments demonstrating both accelerated maturation and apparent rejuvenation in several woody species are discussed with reference to several maturation models.