This book explores the profound importance of phenotypic plasticity as a central organizing theme for understanding biology. Chapters take a broad, integrative approach to explain how physical and biological environmental stimuli (temperature, photoperiod, nutrition, population density, predator presence, etc.), influence insect biochemical, physiological, learning, and developmental processes, altering phenotype, which then influences performance, ecology, life-history, survival, fitness, and subsequent evolution. Topics include endocrinology, development, body size, allometry, polyphenism, reproduction, reproductive and life-history tradeoffs, alternative mating and life-history strategies, density-dependent prophylaxis, physiological adaptation, acclimation, homeostasis, heat-shock proteins, learning, adaptive anti-predator behavior, and evolution of phenotypic plasticity.