The Mid-Infrared ELT Imager and Spectrograph (METIS) is one of three first light instruments on the ELT. It will provide high-contrast imaging and medium resolution, slit-spectroscopy from 3 - 19um, as well as high resolution (R 100,000) integral field spectroscopy from 2.9-5.3µm. All modes observe at the diffraction limit of the ELT, by means of adaptive optics, yielding angular resolutions of a few tens of milliarcseconds. The range of METIS science is broad, from Solar System objects to active galactic nuclei (AGN). We will present an update on the main science drivers for METIS: circum-stellar disks and exoplanets. The METIS project is now in full steam, approaching its preliminary design review (PDR) in 2018. In this paper we will present the current status of its optical, mechanical and thermal design as well as operational aspects. We will also discuss the challenges of building an instrument for the ELT, and the required technologies.