Environmentally influenced variation in phenotypic expression or phenotypic plasticity is a fundamental property of organisms with consequences for developmental and ecological genetics, evolutionary biology, population and community ecology, conservation biology, and medicine. This chapter begins with definitions and distinctions that identify ways to conceive of the environment and of plasticity. It then discusses these issues: (1) What causes plasticity? What types of genetic architecture and signal transduction mechanisms underpin plastic responses to environment? (2) What is the nature of genetic variation for these responses? (3) How do plastic responses, expressed within a single generation, interact with the slowly changing developmental frameworks characteristic of entire clades? (4) What are the consequences of plasticity for populations and communities? The chapter aims to present a constructive guide to key issues of phenotypic variation, rather than a comprehensive review of a vast field.