This chapter provides a general introduction to phenotypic plasticity by presenting key concepts, using simple examples and comprehensive, up-to-date references. A background section covers the norm of reaction; adaptive and maladaptive plasticity; canalization versus plasticity; and eco-devo (ecological developmental biology). The chapter briefly explains the biology that underlies statistical genotype-by-environment interaction along with analytical, interpretive, and selective implications, including the question of evolutionary potential in novel environments. A discussion of environmental cues and their possible transduction pathways provides background for understanding diverse systems of plasticity and offers guidance for designing plasticity experiments and choosing focal aspects of response. In a section addressing inherited effects of parental environment on offspring phenotypes, the chapter introduces the more controversial topic of transgenerational plasticity and points to questions regarding its potential adaptive impact and epigenetic transmission dynamics. The chapter closes by proposing that both previous and current conditions contribute to individual plasticity in an integrated way, and framing a set of productive avenues for future research.