The atomic nucleus is the site of fundamental processes in nature and the origin of the chemical elements. A better understanding of these phenomena requires knowledge of the nuclear environment, which exhibits single-particle, few-particle, and many-body degrees of freedom. Recent ab initio shell model calculations have succeeded in describing the lightest isotopes using effective single- and few-particle interactions. However, the collective (many-body) effects in heavy nuclei are often modeled using the concept of a nuclear shape. In this talk, I will present a simple picture of nuclear shapes and introduce the phenomenon of shape coexistence in atomic nuclei. After an abbreviated tour of shape coexistence across the nuclear mass surface, I will focus on the transitional nuclei near neutron number N=90, where recent experimental work has begun to explain the rapid onset of nuclear deformation that occurs in this region.