  This paper presents the specification, design, and development of the Radial
Velocity Spectrometer (RVS) on the European Space Agency's Gaia mission.
Starting with the rationale for the full six dimensions of phase space in the
dynamical modelling of the Galaxy, the scientific goals and derived top-level
instrument requirements are discussed, leading to a brief description of the
initial concepts for the instrument. The main part of the paper is a
description of the flight RVS, considering the optical design, the focal plane,
the detection and acquisition chain, and the as-built performance drivers and
critical technical areas. After presenting the pre-launch performance
predictions, the paper concludes with the post-launch developments and
mitigation strategies, together with a summary of the in-flight performance at
the end of commissioning.
