Fiber-based wearable electronic circuits and systems are a very promising next-generation technology for human-computer interaction, long-term health monitoring, virtual reality, and other fields. The advancement of nanotechnology has made it feasible to build electronic devices directly on the surface or inside of single fibers, which have typical thickness of several to tens of microns. However, imparting electronic functions to porous, highly deformable, and three-dimensional fiber assemblies and maintaining them during wear represent great challenges from both views of fundamental understanding and practical implementation. This chapter primarily focuses on the elementary electronic devices of fiber-type transistor, as well as fiber-based transistor circuits. As an important tie between the bottom physical layer to the top system layer, the electronic interfaces of fiber-based elements and arrays have been also considered. Lastly, two prototypes of fiber-based wearable system, the intelligent footwear system and smart cushion cover, are confronted, including the scheme plan, package, testing, and evaluation. Limitations of current materials, fabrication techniques, circuits, and systems concerning manufacturability and performance as well as scientific understanding that must be improved prior to their wide adoption have been also discussed.