Gravitational waves have a periodic effect on the apparent positions of stars on the sky. This effect can be quantified and hence ultra-precise astrometric measurements (like the ones from Gaia) can provide a new method to search for gravitational signals. I will describe the principles which give rise to the astrometric signature of gravitational waves, and examine this result in the context of Einsteinian and alternative polarization states. I will discuss some of the data analysis challenges that will have to be overcome when trying to search for GWs in the extremely large (>10^9 stars) Gaia data set, and will present some preliminary estimates of the sensitivity that may be achievable. I will also describe the significance of astrometric measurements for probing stochastic GW backgrounds, and derive the relevant response correlation functions for all polarization modes. Throughout, I will keep a parallel between our work and analogous results from the PTA community.
**Instructions for how to get to building NW22: Please use the front entrance of NW22. As you come in, the door on the right leading to LIGO lab will be unlocked on the days when GRITTS seminar is scheduled in LIGO lab. Please use the stairs to get to the second floor. As you enter interaction area, the conference room 258 will be on your right.