Smart materials and responsive systems are materials that can change their properties or behavior in response to external stimuli such as temperature, light, pressure, or electric fields. Examples include shape-memory alloys, which "remember" a pre-set shape and return to it when heated, and stimuli-responsive polymer that alter their form or function in response to environmental changes. These materials are used in applications such as self-healing structures, adaptive sensors, wearable devices, and robotics, enabling systems that can react, adapt, and function autonomously in dynamic environments.