This chapter elaborates the biosensors in health care for the 21st century. A flexible conductimetric sensor was developed for biological fluids. This sensor was constructed in a sandwich configuration with a hydrophilic membrane placed between two gold deposited layers. The sensor was attached to a contact lens, and placed directly onto the surface of a rabbit eye to monitor the electrical conductivity of tear fluids. An arm-band type biosensor was constructed using a lactate electrode. This biosensor can be placed directly onto the skin, and used to monitor the lactate in sweat. The lactate concentration in sweat was found to be linearly related to the ammonium ion concentration. The average concentrations of lactate, and ammonium ion measured in sweat induced by exhaustive exercise were much higher than those obtained from heat induced sweat. This fact demonstrates that the analysis of lactate ion in sweat using this type of biosensor is an effective method for the estimation of physical condition.