The shift from traditional laboratory analysis to point-of-care testing is driving the evolution of modern diagnostic devices toward wearable electrochemical sensors. This chapter offers an overview of the foundational electrochemical principles crucial for understanding the operation of on-body sensors. Various electrochemical techniques, including amperometry, voltammetry, potentiometry, and electrochemical impedance spectroscopy, are discussed to elucidate their roles in advancing wearable sensor technology. Additionally, a variety of wearable biosensor platforms such as patches, textile-based sensors, microneedle-based sensors, and sensors integrated into contact lenses and eyeglasses are explored. By exploring the electroanalytical principles that underpin these devices, we provide insight into their design and functionality. While this chapter focuses primarily on electrochemical sensors, it is important to note that wearable sensors encompass a broad spectrum of modalities, including optical sensors such as heart rate or oxygen saturation, mechanical sensors for motion or pressure monitoring, and thermal sensors for skin temperature monitoring. These alternative sensing mechanisms complement electrochemical methods and expand the range of applications for wearable devices. To facilitate reader comprehension, we include questions and answers at the end of the chapter, fostering critical thinking, summarization, and potential avenues for further research.