  The Event Horizon Telescope (EHT) is a very long baseline interferometry
(VLBI) array that comprises millimeter- and submillimeter-wavelength telescopes
separated by distances comparable to the diameter of the Earth. At a nominal
operating wavelength of ~1.3 mm, EHT angular resolution (lambda/D) is ~25
micro-as, which is sufficient to resolve nearby supermassive black hole
candidates on spatial and temporal scales that correspond to their event
horizons. With this capability, the EHT scientific goals are to probe general
relativistic effects in the strong-field regime and to study accretion and
relativistic jet formation near the black hole boundary. In this Letter we
describe the system design of the EHT, detail the technology and
instrumentation that enable observations, and provide measures of its
performance. Meeting the EHT science objectives has required several key
developments that have facilitated the robust extension of the VLBI technique
to EHT observing wavelengths and the production of instrumentation that can be
deployed on a heterogeneous array of existing telescopes and facilities. To
meet sensitivity requirements, high-bandwidth digital systems were developed
that process data at rates of 64 gigabit/s, exceeding those of currently
operating cm-wavelength VLBI arrays by more than an order of magnitude.
Associated improvements include the development of phasing systems at array
facilities, new receiver installation at several sites, and the deployment of
hydrogen maser frequency standards to ensure coherent data capture across the
array. These efforts led to the coordination and execution of the first Global
EHT observations in 2017 April, and to event-horizon-scale imaging of the
supermassive black hole candidate in M87.
