Smart polymers are a unique class of advanced materials that are capable of responding dynamically to external stimuli such as temperature, pH, light, magnetic or electric fields, and even biological signals. Their reversible behavior and high sensitivity make them ideal for use in sensor systems across various applications like medical, environmental, and industrial domains. This chapter introduces the fundamental principles of smart polymers and traces their development from basic mechanical materials to bioresponsive, multifunctional components. The key focus is to various types of smart polymers with context to temperature-sensitive, pH-sensitive, light-sensitive, bioresponsive, and field-responsive, which highlight their mechanisms of action and specific sensing functions. The chapter also explores their integration into devices for environmental monitoring, particularly water quality assessment, where they detect contaminants and physicochemical changes with high specificity. The potential utilization of such sensors extends to applications in drug delivery, wearable technology, and soft robotics. Considering the demands of our time and the need for effective utilization, smart polymers are positioned as critical enablers in the advancement of intelligent, adaptive systems for real-time sensing and responsive intervention.