Flexible electronics and particularly soft fibers and textiles are becoming key components in rapidly developing fields such as robotics, health and personal care, sensing, or implantable and wearable devices. Thus far however, the ability to impart soft and stretchable fibers with advanced electronic functionalities, including efficient energy harvesting, remains limited. The recent development of the concept of multi-material fiber drawing has opened novel opportunities in the integration of novel materials and their organization into complex architectures, resulting in unexpected functionalities. Yet, two limitations have resisted this line of research: using fiber-based constructs for advanced microfluidics, and demonstrating state-of-the-art fibers and textiles for energy harvesting and self-powered sensing. In this thesis, via a combination of fundamental understanding and engineering of materials processing, rheology, mechanical and electrical engineering, we significantly advanced the range of materials, architectures and functionalities achievable for multi-material electronic fibers. We in particular achieved three main objectives. First, we demonstrate a uniform capillary-like fiber that integrates an encapsulated micro-channel and an embedded capacitor system within a polymeric cladding. It shows versatile functionalities applicable to microfluidic sensing including the monitoring of the presence and travel distance of a fluid, real-time sensing of a wide range of flowrate, and high-accuracy identification of the static dielectric constant of fluid. We then demonstrate the scalable fabrication of advanced triboelectric fibers that combine a micro-textured elastomer surface with the integration of several liquid metal electrodes. Such fibers exhibit high efficiencies on par with planar systems regardless of repeated large deformations. They can be woven into machine-washable textiles with high electrical outputs up to 490 V, 175 nC. We also demonstrate their self-powered breathing monitoring and gesture sensing capabilities. The third objective arose in the course of the second project ̶ attempting to circumvent the limitations associated with single-electrode triboelectric fibers and unravel more activation states of the devices. We design and fabricate a stretchable multi-material contact-separation mode triboelectric fiber that can generate electricity from reversible compression and stretching. The fiber is constituted of two stretchable and conductive composite-based micro-structured triboelectric parts, which are separated by a large gap and surrounded by a water repellent elastomeric cladding. Finite element analysis is performed to deeply understand the kinetic deformations of the fiber system, and show the level of control and engineering of the mechanical behaviors that can be achieved through application-targeted fiber designs. The novel triboelectric fiber designs that we propose, the unprecedented simplicity and throughput of the manufacturing process, together with the inherent deformability and excellent electrical outputs of the demonstrated fibers and textiles, alleviate the challenges associated with the integration of high performance energy-harvesting systems within soft and wearable constructs. With novel sensing and energy harvesting soft fibers, the results of this thesis pave the way towards novel devices and applications in health and personal care, implantable systems and wearable devices.