Polystemonous androecia are diverse in both number and position of stamens. This investigation of polystemonous Hydrangeaceae uses developmental data to characterize (1) the range of developmental variations that account for the diverse androecial patterns and (2) how the expressions of polystemony among Hydrangeaceae compare to those found generally among other angiosperms and especially in their sister family, the Loasaceae, some of which have particularly complex androecia. All polystemonous Hydrangeaceae share the common element of stamen clusters in antesepalous positions. In each of these taxa, the first stamens are initiated opposite the medians of the sepals. Subsequently, stamens form laterally on the flanks of the initial antesepalous stamens, giving rise to the clusters designated as antesepalous triplets. The simplest elaborations based on those common initial developmental steps include (1) adding additional lateral flanking stamens and (2) adding a single stamen in each antepetalous position between adjacent antesepalous groups. More complex elaborations are characteristic of (1) Carpenteria and Philadelphus, which form common primordia at the beginning of androecial development and, subsequently, have stamen primordia form on them, and (2) Deinanthe, which has an elongate hypanthial region on which numerous whorls of stamens are initiated. Carpenteria is unique among Hydrangeaceae in having groups of stamens that are initiated centrifugally in antepetalous positions, and this is similar to complex elements found among some Loasaceae. Generally, the polystemony of Hydrangeaceae that is based in the formation of antesepalous triplets is very similar to that found to evolve in parallel among various clades of rosids and asterids.