The spatial and temporal distributions of plant hormones during tree-ring formation are beginning to be elucidated. Plant hormones play specific roles in tree-ring formation depending on the developmental zone in which they operate. Tree rings are the result of the seasonal activity of the vascular cambium, the secondary meristem of woody angiosperms and gymnosperms. During tree-ring formation, a combination of endogenous and environmental factors affects cambial division, cell differentiation and maturation; this, in turn, affects wood quantity and quality. Among the endogenous factors affecting tree-ring formation, plant hormones are recognized as determinant players in regulating many aspects of the features and fate of each xylem cell. Most of our knowledge regarding the roles of plant hormones on tree-ring formation comes from herbaceous plants, although an increasing interest involves the analysis of the hormonal patterns in tree-species. This paper reviews the state of knowledge of the role of plant hormones during tree-ring formation by focusing on experiments performed on woody species. An overview of the main plant hormones and their main activities during radial tree growth will be followed by discussion of their role in each tree-ring developmental stage and in the overall tree-ring seasonal pattern.