  Direct detection experiments are poised to detect dark matter in the form of
weakly interacting massive particles (WIMPs). The signals expected in these
experiments depend on the ultra-local WIMP density and velocity distribution.
Firstly we review methods for modelling the dark matter distribution. We then
discuss observational determinations of the local dark matter density, circular
speed and escape speed and the results of numerical simulations of Milky
Way-like dark matter halos. In each case we highlight the uncertainties and
assumptions made. We then overview the resulting uncertainties in the signals
expected in direct detection experiments, specifically the energy, time and
direction dependence of the event rate. Finally we conclude by discussing
techniques for handling the astrophysical uncertainties when interpreting data
from direct detection experiments.
