Abstract: A computer built entirely using transistors based on carbon nanotubes, which is capable of multitasking and emulating instructions from the MIPS instruction set, is enabled by methods that overcome inherent challenges with this new technology. Carbon nanotubes have long been touted as promising building blocks for computers based on carbon rather than silicon. A main motivation towards this goal is the potential for circuits using carbon nanotube transistors to achieve high energy efficiency. Various carbon nanotube electronic circuit blocks have been demonstrated previously, but Max Shulaker et al. now reach a true milestone in the fields of carbon electronics and nanoelectronics by building a simple but functional computer made entirely from carbon nanotube transistors. Composed of 178 transistors, each containing between 10 and 200 carbon nanotubes, it runs a simple operating system and is capable of multitasking: it performs four tasks (summarized as instruction fetch, data fetch, arithmetic operation and write-back) and can run two different programs concurrently. The miniaturization of electronic devices has been the principal driving force behind the semiconductor industry, and has brought about major improvements in computational power and energy efficiency. Although advances with silicon-based electronics continue to be made, alternative technologies are being explored. Digital circuits based on transistors fabricated from carbon nanotubes (CNTs) have the potential to outperform silicon by improving the energy–delay product, a metric of energy efficiency, by more than an order of magnitude. Hence, CNTs are an exciting complement to existing semiconductor technologies1,2. Owing to substantial fundamental imperfections inherent in CNTs, however, only very basic circuit blocks have been demonstrated. Here we show how these imperfections can be overcome, and demonstrate the first computer built entirely using CNT-based transistors. The CNT computer runs an operating system that is capable of multitasking: as a demonstration, we perform counting and integer-sorting simultaneously. In addition, we implement 20 different instructions from the commercial MIPS instruction set to demonstrate the generality of our CNT computer. This experimental demonstration is the most complex carbon-based electronic system yet realized. It is a considerable advance because CNTs are prominent among a variety of emerging technologies that are being considered for the next generation of highly energy-efficient electronic systems3,4.