Microarcsecond astrometry with MCAO using a diffractive mask We present a new ground-based technique to detect or follow-up long-period, potentially habitable exoplanets via precise relative astrometry of host stars using Multi-Conjugate Adaptive Optics (MCAO) on 8 meter telescopes equipped with diffractive masks. MCAO improves relative astrometry both by cancellation of high-altitude atmospheric layers, which induce dynamic focal-plane distortions, and the improvement of centroiding precision with sharper PSFs. However, mass determination of habitable exoplanets requires multi-year reference grid stability of ~1-10 uas or nanometer-level stability on the long-term average of out-of-pupil phase errors, which is difficult to achieve with MCAO (e.g., Meyer et al. 2011). The diffractive pupil technique calibrates dynamic distortion via extended diffraction spikes generated by a dotted primary mirror, which are referenced against a grid of background stars (Guyon et al. 2011). We show that the diffractive pupil improves MCAO's long-term astrometric stability to the microarcsecond level. The diffractive grid provides three other benefits for relative astrometry: (1) increased dynamic range, permitting observation of V < 10 stars without saturation; (2) calibration of dynamic distortion; and (3) a spectrum of the target star, which can be used to calibrate the magnitude of differential atmospheric refraction to the microarcsecond level. A diffractive 8-meter telescope with MCAO reaches < 3-5 micro-arcsec relative astrometric error per coordinate in one hour on a bright target star in fields of moderate stellar density (~10-40 stars arcmin-2). We also present recent results from an on-sky test of the diffractive mask on the Nickel 1-m telescope.