This book discusses the advances in sensor technologies and sensing efficiency. It highlights different sensor applications, including humidity, gas, fluorescent, biological, optical, radiation, etc. The chapters discuss recycled and biodegradable materials-based sensors as well as sensing techniques and theories. The different approaches employed to modify the electrode surfaces of sensors to lower the overpotential, enhance sensitivity to enrich the desired species and/or lessen the influence of interferences are also covered. This handbook is structured in seven sections including fundamentals of sensor technologies, types of sensors, and medical, biological, environmental, and industrial applications of sensors.