Sensory systems are available today at all length scales and formulate an important mandate in sustainability of environment and health. They use a wide spectrum of transduction mechanisms and signal conversion approaches with different levels of accuracy and rapidity. Sensors as deployed in various ranges of applications span from relatively simple temperature measurement bimetallic thermocouple structures to the detection of specific entities using advanced physical principles. The best example of sensors emanates from nature itself. Almost all living beings are blessed with sensory systems to sense and act to various environmental stimuli. There is a lot of inferential learning from such systems which can be translated to modern day sensor research. Within the environmental, chemical, and medical domains sensing can be carried out across a variety of length scales like the macroscale, microscale, or nanoscale. In a very organized manner a sensory system can be simplified into an analyte of interest (external to the sensor), a detection element (which is fixated to the sensory surface), a single transduction mechanism (to record measurable signal coming out from the change of analyte concentration), an analyzer and a decision tool. Further, the sensory systems can be using mechanical, microelectronic, micromechanical or electromechanical, optical, electrochemical, colorimetric, and other means to perform rapid sensing in the physical, chemical, and biological types of analytes. This book describes the basic mechanisms, fabrication techniques, and recent advancements in developments related to environmental, chemical, and medical sensors.