This chapter summarizes the breadth of morphological variability within and among species and indicates which morphological characters are particularly useful taxonomically within the genus. There is a striking range of morphological types ranging from tiny, prostrate herbs to suffruticose shrubs, and erect herbs over two meters tall. Most of the species have flowering heads that are yellow or yellow-orange, but three species have white ray flowers (the white-rayed complex). Considerable discussion is devoted to features of the fruits, which can often be diagnostic at a specific level. An outline of the classification of the genus is provided to serve as a guide for discussions in the other chapters.