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The Event Horizon Telescope (EHT) is a very long baseline interferometry(VLBI) array that comprises millimeter- and submillimeter-wavelength telescopesseparated by distances comparable to the diameter of the Earth. At a nominaloperating wavelength of ~1.3 mm, EHT angular resolution (lambda/D) is ~25micro-as, which is sufficient to resolve nearby supermassive black holecandidates on spatial and temporal scales that correspond to their eventhorizons. With this capability, the EHT scientific goals are to probe generalrelativistic effects in the strong-field regime and to study accretion andrelativistic jet formation near the black hole boundary. In this Letter wedescribe the system design of the EHT, detail the technology andinstrumentation that enable observations, and provide measures of itsperformance. Meeting the EHT science objectives has required several keydevelopments that have facilitated the robust extension of the VLBI techniqueto EHT observing wavelengths and the production of instrumentation that can bedeployed on a heterogeneous array of existing telescopes and facilities. Tomeet sensitivity requirements, high-bandwidth digital systems were developedthat process data at rates of 64 gigabit/s, exceeding those of currentlyoperating cm-wavelength VLBI arrays by more than an order of magnitude.Associated improvements include the development of phasing systems at arrayfacilities, new receiver installation at several sites, and the deployment ofhydrogen maser frequency standards to ensure coherent data capture across thearray. These efforts led to the coordination and execution of the first GlobalEHT observations in 2017 April, and to event-horizon-scale imaging of thesupermassive black hole candidate in M87.

First M87 Event Horizon Telescope Results. II. Array and Instrumentation