This chapter describes various flexible and wearable chemical sensors and their application in noninvasive biomonitoring. Modern bioinstrumentation techniques were developed in response to demands for on-site monitoring. The wearable oxygen sensor is a filmlike version of Clark’s oxygen electrode that is made up of flexible functional polymer membranes such as polypropylene and fluorinated ethylene-propylene, on which film electrodes and the electrolyte layer are formed. The flexible oxygen sensor measures oxygen levels by a two-electrode amperometric method. The transcutaneous partial pressure of oxygen can be monitored using a polarographic technique that is noninvasive and provides a continuous measurement of blood oxygen through the skin. The most promising method for continuous glucose monitoring has been enzyme-based biosensors. Glucose is usually measured by quantifying the production of hydrogen peroxide or the consumption of oxygen by the enzyme glucose oxidase reaction using electrochemical or spectrophotometric methods.