Although phenotypic plasticity is ubiquitous, its causes and consequences remain controversial, especially among evolutionary biologists. This chapter reviews what is known––and what is not known––about some of these causes and consequences by asking five key questions: (1) Does plasticity confer a unique evolutionary advantage? (2) What are the proximate mechanisms of plasticity? (3) When should plasticity evolve, and what form should it take? (4) Can plasticity influence evolution? and (5) Does plasticity fit within existing evolutionary theory? Further research into these and other questions about plasticity promises to provide fresh perspectives into how living things develop, function, interact with each other, and evolve.