  The P2 experiment in Mainz aims to measure the weak mixing angle in electron-
proton scattering to a precision of 0.13 %. In order to suppress uncertainties
due to proton structure and contributions from box graphs, both a low average
momentum transfer $Q^2$ of $4.5\cdot 10^{-3}$ GeV$^2/c^2$ and a low beam energy
of 155 MeV are chosen. In order to collect the enormous statistics required for
this measurement, the new Mainz Energy Recovery Superconducting Accelerator
(MESA) is being constructed. These proceedings describe the motivation for the
measurement, the experimental and accelerator challenges and how we plan to
tackle them.
