Implantable electronic biomedical devices are used clinically to diagnose and treat an increasing number of medical conditions. Such devices typically employ hermetic packages that often incorporate electrical feedthroughs made with conventional ceramic-to-metal bonding technologies. This sealing technology is well established and provides robust hermetic seals, but is limited in both the number and spacing of electrical leads. Emerging devices for interfacing with the human nervous system, however, will require a large number of external electrical leads implemented in a miniaturized packaging configuration. Commercially available feedthrough technologies are currently incapable of providing external electrical contacts with spacings as small as 200 to 400 microns, and thus are neither compatible with integrated circuit I/O (input/output) pad spacings nor with miniature implantable packages. We report the development of a hermetic high-density feedthrough (HDF) technology that allows for conductive path densities as high as 1,000 per cm2, and that is capable of supporting neural interface devices. The fabrication process utilizes multilayer high temperature co-fired ceramic (HTCC) technology in conjunction with platinum leads. Before co-firing, green alumina substrates are interleaved with linear, parallel Pt trace arrays in either wire or thin foils to form the electrical feedthroughs. Layered stacks of spatially isolated traces are first compacted into a composite, and then fired to achieve densification. After firing, the densified multilayered composite compacts are sliced perpendicular to the Pt traces and lapped to produce multiple feedthrough arrays with a high density of leads (conductors). Both hermeticity and biocompatibility of such implantable feedthroughs are important, as both moisture and positive mobile ion contamination from the saline environment of the human body can lead to compromised performance or catastrophic failure. HDFs fabricated using this process with 100 conductors and lead-to-lead spacings as low as 400 microns have been helium leak tested repeatedly and found to exceed industry-accepted standards with helium leak rates in the range of 10–11 mbar-l/s. The spacing of the current prototype matches industry standard neural interface technology, and can be scaled to higher densities with lead-to-lead spacings as small as 200 microns. The reported HDF process has several distinct advantages over prior approaches, including the provision of a large number of conductive feedthrough leads suitable for flip-chip bonding with sub-mm lead-to-lead spacings (pitch), and the incorporation of materials (alumina and platinum) that are already used in medical implants. The implementation of such an HDF technology allows for significant package miniaturization, allowing greater flexibility in surgical placement as well as less invasive procedures for implantable electronic biomedical devices.