Global climate change and human-driven actions are altering climatic conditions and habitat characteristics worldwide. As a result, animals experience various environmental changes that occur simultaneously, requiring them to cope daily with multidimensional environmental variation. A key approach to addressing the fundamental question of whether and how populations can successfully adapt to environmental variability is to study how organisms respond to environmental changes by modifying their physiology and the consequences of these responses. Traits such as hormones and metabolic rates are not only highly sensitive to environmental fluctuations but also inherently flexible, allowing for rapid responses that vary across different levels—from species to populations, to individuals, and even within individuals. In this talk, I first describe how the responsiveness of physiological traits to environmental changes, known as phenotypic plasticity, can be measured by quantifying the degree of change via the slope of their reaction norm. Second, I use data from a captive songbird population that I repeatedly exposed to a simulated cold spell, to demonstrate that important information can be gained by quantifying variation in plasticity both among and within individuals in key physiological traits, such as plasma concentrations of corticosterone and rates of organismal metabolism. Third, I highlight the evolutionary implications of such variation in reaction norm slopes by reviewing available evidence that plasticity plays a crucial role in enabling evolutionary adaptation, including insights from my own work. Finally, I conclude by describing promising areas for future research and argue that studying variation in physiological plasticity is crucial for assessing the viability of species and populations in our ever-changing world.