Abstract-Very recently, a new empty coaxial structure, entirely built with printed circuit boards, has been proposed. The resulting coaxial line has low radiation, low losses, high quality factor, mono-mode propagation and is non-dispersive. Also, and conversely to other coaxial line designs, the fabrication process of this empty coaxial line permits to easily adjust the dimensions of the inner and outer conductors. Thus, the response of the coaxial line can be tailored by designing the shape of the conductors appropriately. Up to now, this novel coaxial line has not been successfully integrated in a planar substrate, since a working transition to a traditional planar line has not been defined yet. Therefore, in this paper, a novel and high-quality transition from coplanar waveguide to this new and versatile empty coaxial line is proposed. With this transition, the coaxial line is completely integrated in a planar circuit board, so that it truly becomes an empty substrate integrated coaxial line. The proposed transition has been fabricated and assembled. Both full-wave simulated and measured results show an excellent agreement. Therefore, the proposed transition is suitable to develop completely substrate integrated planar components for applications in wideband communication systems that require very high-quality responses and protection from external electromagnetic interferences. In order to show this fact, this new transition has been applied to integrate a high-performance empty coaxial line filter in a planar substrate. The measured response of this filter is excellent, and proves the goodness of the proposed transition that has enabled, for the first time, the complete integration of an empty coaxial line in a planar substrate.Index Terms-Empty substrate integrated coaxial line, ESICL, substrate integrated coaxial line, SICL, substrate integrated waveguide, SIW, substrate integrated circuit, SIC, bandpass filter.