We present here SPECULOOS, a new exoplanet transit search based on a network of 1m-class robotic telescopes targeting the 1200 ultracool (spectral type M7 and later) dwarfs bright enough in the infrared (K-mag <= 12.5) to possibly enable the atmospheric characterization of temperate terrestrial planets with next-generation facilities like the James Webb Space Telescope. The ultimate goals of the project are to reveal the frequency of temperate terrestrial planets around the lowest-mass stars and brown dwarfs, to probe the diversity of their bulk compositions, atmospheres and surface conditions, and to assess their potential habitability.