This is the second part of the three-volume set monograph Peonies of the World, a comprehensive taxonomic revision of the genus Paeonia. Part of Hong De-Yuan's extensive field work on the distribution of the genus Paeonia, and supported by the National Geographical Society, Peonies of the World, Volume 2: Polymorphism and Diversity contains 356 colour photographs, taken from natural sites, covering all 32 species and 26 subspecies, accompanied with concise descriptions of morphology and distribution. The photographs illustrate the extraordinary variations of morphology, and colour in particular, within individual populations and species, and the uniformity of others. Hong De-Yuan's knowledge of polymorphism and diversity will be immensely valuable in teaching population biology and biodiversity. It is also an object lesson for herbarium taxonomists to be cautious when revising existing taxa or describing new taxa based purely on form. This second volume of Peonies of the World will be of enormous value to horticulturists and plant geneticists breeding new peony cultivars, and is destined to become the definitive guide to species for growers, gardeners and collectors.