Recent strong scientific and technological interest in nanostructured carbon materials (nanocarbons) has been motivated by the diverse range of physical properties these systems exhibit. These properties arise from the many different local bonding structures of carbon, as well as the long-range order. For example, carbon nanotubes (CNTs) are distinct from graphite, although both consist essentially of sp 2 -bonded carbon; [1] CNTs are the strongest known material and exhibit unique electronic transport properties, making them candidates for a wide range of applications. Similarly, nanocrystalline diamond films are distinct from single-crystal diamond, although both are mostly sp 3 -bonded carbon, and exhibit high hardness, exceptional chemical inertness, biocompatibility, [2] and negative electron affinity with proper treatment.[3]The unique mechanical and electrochemical properties of nanocrystalline diamond make it a promising candidate for use as a protective coating of machining tools; a hermetic, corrosion-resistant coating for biodevices; [4] a cold-cathode electron source; [5] and structural material in micro-and nanoelectromechanical systems (MEMS/NEMS).[6]It has been speculated that the synergistic combination of carbon nanotubes and nanocrystalline diamond could give rise to materials with novel properties that could be used advantageously in applications such as electronic devices [7] or MEMS/NEMS. [8] However, a synthesis pathway must be developed that would lead to the concurrent growth of different allotropes of carbon that are covalently bonded and organized at the nanometer-scale. In this work we demonstrate, for the first time, that the simultaneous synthesis of nanocrystalline diamond and carbon nanotubes to form a covalently bonded hybrid materialÐa nanocomposite of diamond and CNTsÐ can be achieved. One of the most commonly used processes for preparing nanostructured carbon materials is plasma-enhanced chemical vapor deposition (PECVD), in which chemically activated carbon-based molecules are produced. For instance, different carbon-rich combinations of C 2 H 2 /H 2 , [9] C 2 H 2 /NH 3 , [10,11] and CH 4 /Ar [12] have been employed for growing CNTs. In contrast, hydrogen-rich (~99 % H 2 ) CH 4 /H 2 plasmas are the most common mixtures used for growing microcrystalline diamond films, wherein large amounts of atomic hydrogen play a critical role in both the gas-phase and surface-growth chemistries. Importantly, atomic hydrogen is also needed to selectively etch the non-diamond carbon during growth. Over the past several years we have developed hydrogen-poor Ar/CH 4 (99 % Ar, 1 % CH 4 ) chemistries to grow ultrananocrystalline diamond (UNCD) films, which consist of diamond grains 3±5 nm in size, and atomically abrupt high-energy grain boundaries.[8] The special nanostructure of UNCD yields a unique combination of properties, such as low deposition temperatures, [13] excellent conformal growth on high-aspect ratio features, [8] and the highest room-temperature n-type electronic conductivity demons...