In this talk I will summarize the main characteristics and scientific rationale behind MEGARA, a R=6000-20000 optical IFU and Multi-Object Spectrograph that will be installed at GTC on 2015. The MEGARA IFU will allow fully covering a region of 14x12 arcsec2 while the MOS will allow observing up to ~100 objects in 3.5x3.5 arcmin2 with a reconfiguration time of ~1 min. This, along with the large collecting area of GTC and an optimized control system, will allow performing spectroscopic surveys with unprecedented high efficiency both in clusters (of all kinds) and in the field, especially when very deep limiting fluxes are aimed for.