  An Atmospheric Dispersion Corrector (ADC) uses a double-prism arrangement to
nullify the vertical chromatic dispersion introduced by the atmosphere at
non-zero zenith distances. The ADC installed in the Gemini Planet Imager (GPI)
was first tested in August 2012 while the instrument was in the laboratory. GPI
was installed at the Gemini South telescope in August 2013 and first light
occurred later that year on November 11th. In this paper, we give an overview
of the characterizations and performance of this ADC unit obtained in the
laboratory and on sky, as well as the structure of its control software.
