A Raman lidar dedicated to night-time tropospheric water-vapor high-resolution measurements is currently being developed at Réunion island in the south-western Indian Ocean. To our knowledge, it is the first permanent instrument of its kind in this tropical region. The geophysical and instrumental interests and issues on the radiative, dynamical and chemical plans for such a measurement, specially in the tropics, are obvious. The choice of a visible laser excitation wavelength was initially a constraint, in view of the weakness of the Raman scattering process that is the basis of the development of this instrument, but many arguments also plead for such a choice. After describing the water-vapor measurement method of this lidar, which is straightforward in principle, we stress on the main delicate underlying issues related to this method. A precise description of the optical parts of the lidar system is then given that emphasizes the importance of the rejection of the elastically backscattered signals in the Raman channels. Finally, we list the most important future works concerning the validation and calibration stages of this instrument that is intended to become an atmospheric surveillance instrument on a medium term.