The instrumentation in Hall A at the Thomas Jefferson National Accelerator Facility was designed to study electro- and photo-induced reactions at very high luminosity and good momentum and angular resolution for at least one of the reaction products. The central components of Hall A are two identical high resolution spectrometers, which allow the vertical drift chambers in the focal plane to provide a momentum resolution of better than 2×10 −4. A variety of Cherenkov counters, scintillators and lead-glass calorimeters provide excellent particle identification. The facility has been operated successfully at a luminosity well in excess of 10 38 cm −2 s −1 . The research program is aimed at a variety of subjects, including nucleon structure functions, nucleon form factors and properties of the nuclear medium.