Flexible electronics and sensors are a key enabler for current technology trends such as Industry 4.0 or the Internet of Things. Particularly, these flexible devices can be used for the realization of geometry adaptive devices, wearables or electronics that can be embedded into light weight structures. We present a new approach for the fabrication of an ultra-thin and highly flexible printed circuit board (PCB) with a total thickness of only 20 µm or even less. This PCB bases on the thermoplastic polymer Parylene, which combines a variety of unique properties such as optical transparency, biostability and biocompatibility according to ISO 10993, thermal stability as well as low permeability for gases and water. For the realization of ultra-thin flexible PCBs, Parylene was used as a substrate, a dielectric between the metallic redistribution layers (RDL) as well as for the encapsulation. The RDL were deposited and patterned by standard microsystem technologies. Hence, structure sizes of down to 10 µm were successfully realized. Using the different metallization approaches, multiple RDL can be realized, which was tested for up to three RDL, but is scalable to even more. For the contact formation through the vias, different methods were investigated, which include sputtering, printing and electrochemical deposition. The electrical properties were found to be excellent for flat and bent conditions. Besides the fabrication of the ultra-thin PCB itself, the integration of electronic and sensory components on this PCB is crucial. Doing so, the direct fabrication of components on the PCB was investigated and successfully demonstrated for a pH sensor and a chemical sensor for volatile organic compounds. In parallel, for the integration of discrete devices, several technologies are under investigation, such as soldering, wire bonding, and printing of conductive adhesives. Utilizing the unique properties of Parylene, also completely new integration approaches can be realized, which include the embedding of electronic components and the transfer of components from rigid carrier substrates. Besides the fabrication and integration technologies, a third aspect is the scalability to mass fabrication. Addressing this, a concept for the roll-to-roll deposition of Parylene was developed and successfully verified in a test setup.