The needs of emerging clinical applications of neural stimulation and recording in the treatment of many diseases and disorders are driving a reduction in the size of implanted devices and the development of strategies to manage the large number of electrical interconnects between implanted electrodes and control electronics. The requirements for large numbers of electrodes, exceeding many hundreds for vision prostheses, and the ability to interface with nerves that may be as small as 100 microns in diameter, precludes the use of conventional packaging with hermetically sealed metal or ceramic cans. One of the few practical alternatives to hermetically sealed enclosures is the use of thin-film dielectrics, possibly combined with polymer over-layers. Besides the need for these thin-film coatings to provide effective passivation against corrosion, they also serve a multifunctional role providing adhesion between polymer and metal layers in flexible devices, providing a biocompatible interface to neural tissue and in some devices, providing a surface for functionalization with bioactive molecules. Conventional passivation materials such as silicon dioxide and silicon nitride are prone to corrosion in vivo and recent experience with polymer encapsulation such as Parylene-C suggests that this otherwise excellent barrier layer may fail after chronic implantation for more than about one year. The implanted electronics on these devices operates typically at 3 V or higher and neural stimulation involves pulsatile currents that may also induce unexpected failures at interfaces. In this context, other thin-film materials such as amorphous silicon carbide (a-SiC), ultra-nanocrystalline diamond (UNCD), and atomic-layer-deposition (ALD) Al2O3 are being investigated as alternative passivation materials that can provide chronic protection of active implants. A discussion of the physical and chemical requirements for these materials and results reported to date is presented. Emphasis is placed on understanding the constraints of the application including processing compatibility with temperature-sensitive substrates, the need for conformal coatings, and appropriate test methodologies to validate predictions of chronic in vivo lifetimes. Early results are promising with some combinations of thin-film and polymer encapsulation exhibiting excellent stability and biocompatibility. Of particular interest, are the relative roles and importance of interfacial properties and bulk barrier properties in achieve long-term chronic passivation of implanted active devices.