The constant miniaturization of electronics and the development of powerful data processing tools have enabled the fast advance of wearable technology. The successful demonstration of wearable devices for sports monitoring has already been made. Although such outstanding technology has shown great performance, some important features are still missing. Nowadays, wearable devices are limited to monitoring physical parameters such as motion, temperature, heart rate, steps, etc., while important (bio)chemical information, such as metabolites (glucose, lactate, alcohol) and electrolytes (sodium, potassium) levels can only be accessed via blood analysis. The development of on-body detectors for physiological self-monitoring has profound societal implications for healthcare, nutrition analyses, and remote medicine. Electrochemical wearable biosensors are promising candidates to bridge the technology gap for wearable devices toward comprehensive monitoring and measurement of important, and medically relevant, biological changes in the human body. Electrochemical sensors can be easily miniaturized, data can be accessed wirelessly, and the concept of "put-and-forget" can be successfully employed minimizing the hassle and interference in the wearer's daily-life activities. In this dissertation, the development and application of non-invasive wearable devices are discussed. Wearable electrochemical sensors operating in sweat, saliva, tears, and interstitial fluid are presented in individual chapters with a final chapter on the critical discussion regarding the challenges and opportunities involved in building a successful electrochemical wearable biosensor capable of reliable and trustful measurement readings. The criteria for analytical biomarker detector in saliva, sweat, tears, and interstitial fluid are included.