  We describe the design and deployment of GREENBURST, a commensal Fast Radio
Burst (FRB) search system at the Green Bank Telescope. GREENBURST uses the
dedicated L-band receiver tap to search over the 960$-$1920 MHz frequency range
for pulses with dispersion measures out to $10^4$ pc cm$^{-3}$. Due to its
unique design, GREENBURST will obtain data even when the L-band receiver is not
being used for scheduled observing. This makes it a sensitive single pixel
detector capable of reaching deeper in the radio sky. While single pulses from
Galactic pulsars and rotating radio transients will be detectable in our
observations, and will form part of the database we archive, the primary goal
is to detect and study FRBs. Based on recent determinations of the all-sky
rate, we predict that the system will detect approximately one FRB for every
2$-$3 months of continuous operation. The high sensitivity of GREENBURST means
that it will also be able to probe the slope of the FRB source function, which
is currently uncertain in this observing band.
