We are currently developing a conceptual design for a future ELT NIR large field of view imager-spectrograph, SMART-MOS, along with several science cases from which top level instrument requirements are being established. This project form part of the EU Framework 6 Joint Research Activity on Smart Focal Planes. The instrument will offer multiobject spectroscopy, which will include the long slit mode, and image over a field of view of 1 to 2 arcmin, with high multiplexing capability. The use of a simple AO ground layer correction is envisaged but the instrument will not work at the diffraction limit though. Multi slits are the base line for the field selector, even when multi-IFU type instrument might be considered at later stage. There are two possible operational concepts for SMART-MOS, each of which involving different technologies: MOEMS type devices or sliding bars. We describe in this contribution the science cases and discuss the most relevant requirements which will drive the development. Preliminary optical and mechanical concepts are also sketched which include details about the most relevant part of the instrument.