I will present our survey of planetary nebulae (PNe) in the halo of the Andromeda Galaxy, M31, to constrain its total mass and mass distribution. Our survey, using the INT Wide Field Camera, has obtained [OIII] narrow band and Stromgren Y continuum imaging for a 2.5 square degree area centered on M31 out to a radius of 20 kpc. We now have 1200 PNe candidates for follow-up multi-object spectroscopy; we obtained spectra for ~600 of these in Oct 2001 using Autofib-2 on the WHT, complementing our existing velocity dataset of 294 PNe near the center of M31. Dynamical modelling of our PNe velocities will provide a more accurate measurement of the total halo mass of M31, and will also constrain the velocity anisotropy and mass as a function of radius. Comparison with the predictions of high resolution simulations for minor and major mergers will allow us to assess the importance of merging in the formation of M31.