Smart garments have evolved beyond their traditional functions to serve as wearable digital interfaces. The core of these garments lies in the ability to sense changes in the wearer's body and provide relevant feedback, taking advantage of their close and extensive contact with the human body surface. These garment-friendly sensors aim to support healthcare, entertainment, and lifestyle needs. However, current smart garments still face limitations such as the presence of rigid electronic components, susceptibility to moisture, lack of breathability, and rigid electric insulation or connections prone to deformations. These limitations can hinder wearability, durability, convenience in maintenance, system stability, and safety of the wearable interfaces.To address these limitations, this dissertation highlights three wearable sensor applications utilizing stretchable fiber optics and passive RFID (radiofrequency identification) tags. A respiration sensor used a stretchable fiber optic enclosed by machine embroidery. This sensor tracked changes in thoracic and abdominal circumferences accompanied by breathing behaviors. The machine embroidery secured the optical fiber onto the textile, allowing transition in shapes while providing protection against abrasions. The sensor demonstrated satisfactory results in human tests and maintained sensitivity after multiple machine washes. Another wearable device discussed is a stretchable fiber optic-based insole that monitors plantar pressure. The insole contained a two-dimensional array of soft fibers embedded in a flexible foam. When compressed by the foot, the fibers deformed, leading to decreased light transmittance. Human participant tests showed that the fiber optic insole, coupled with a neural network model, satisfactorily predicted the gait parameters. Furthermore, the system exhibited potential for providing real-time feedback on abnormal gait patterns, such as toe walking in children with autism spectrum disorder (ASD). Lastly, the dissertation introduces the use of passive RFID tag pairs attached to eyeglasses, enabling battery-free head orientation sensing. The signal strength from each tag became different when the person rotated the head in yaw. The system not only correlated well with head movements but also showed potential for measuring rotations in all three axes, which can be utilized in human-computer interactions, including virtual/augmented reality applications for children.