  VIMOS and NIRMOS are two multi Object Spectrographs being developed for the
VLT. Their main scientific characteristic is the high multiplexing capability,
allowing to obtain up to 800 spectra per exposure. To achieve such a high
efficiency, a number of dedicated tools are necessary, from software to
hardware. In part a), we describe the software tools which will be provided to
successful proponents to prepare observations. Paper b) will deal with the
technical aspects of mask manufacturing
