Why is there so much variation in insect behavior? This chapter will address the sources of behavioral variability, with a particular focus on phenotypic plasticity. Variation in social, nutritional, and seasonal environmental contexts during development and adulthood can give rise to phenotypic plasticity. To delve into mechanism underlying behavioral flexibility in insects, examples of polyphenisms, a type of phenotypic plasticity, will be discussed. Selected examples reveal that environmental change can affect gene expression, which in turn can affect behavioral plasticity. These changes in gene expression together with gene-by-environment interactions are discussed to illuminate our understanding of insect behavioral plasticity.