Chemical sensors are devices designed to detect and quantify specific chemical substances or analytes in various environments. They operate based on selective interactions between the target analyte and a sensing element, producing a measurable signal indicative of the analyte concentration. Chemical sensors are utilized in diverse applications such as environmental monitoring, industrial process control, medical diagnostics, and homeland security. They can detect gases, liquids, and solids, ranging from volatile organic compounds to heavy metals. Various sensing principles are employed in chemical sensors, including optical, electrochemical, piezoelectric, and mass-sensitive methods, each offering unique advantages in terms of sensitivity, selectivity, and response time. Miniaturization and integration with microelectronics have enabled the development of portable and wearable chemical sensor devices for on-site and real-time monitoring applications. Advances in nanotechnology and materials science have led to the development of nanomaterial-based sensors with enhanced sensitivity and selectivity. Chemical sensors play a critical role in ensuring safety, quality control, and environmental protection in industries such as pharmaceuticals, food and beverage, and oil and gas. Continuous research and innovation in sensor design, fabrication, and signal processing aim to improve sensor performance, reliability, and cost-effectiveness for a wide range of practical applications. The integration of chemical sensors into Internet-of-Things (IoT) networks and smart systems enables remote monitoring, data analytics, and decision support in various fields. Overall, chemical sensors contribute to enhancing human health, environmental sustainability, and safety by providing timely and accurate information about chemical hazards and pollutants in our surroundings.