When a transiting planet crosses the stellar disk, its opaque body blocks a fraction ( Rp^2 / Rstar^2) of the stellar light and its atmosphere subtly filters an additional fraction. One can thus detect a planet and measure its radius Rp by monitoring for smalls dip in flux in photometric light curves. And one can further measure the wavelength dependence of the planetary radius Rp to detect atomic and molecular features present in the planetary atmosphere. From the ground, photometric light curves have been limited by systematics due to changing air mass, atmospheric conditions, telescope pointing or flat-field errors, that result in a correlated « red » noise. Multi-Object Spectrographs (MOS) allows for simultaneous observations of comparison stars and thus provide an avenue to correct for the atmospheric variations, in each spectral channel. I shall discuss how differential spectro-photometry using MOS can help both the detection and the characterisation of exoplanets. In particular, I shall discuss the ERC-funded project ExTrA which implement this approach to detect Earth-like planets in the habitable zone of cool stars. And I shall discuss how the exoplanets detected by ExTrA could later be characterised using the same technique, with larger telescopes such as the ELT.