  Weakly Interacting Massive Particles (WIMPs) are one of the leading
candidates for Dark Matter. We develop a model-independent method for
determining the mass $m_\chi$ of the WIMP by using data (i.e., measured recoil
energies) of direct detection experiments. Our method is independent of the as
yet unknown WIMP density near the Earth, of the form of the WIMP velocity
distribution, as well as of the WIMP-nucleus cross section. However, it
requires positive signals from at least two detectors with different target
nuclei. In a background-free environment, $m_\chi \sim 50$ GeV could in
principle be determined with an error of $\sim 35%$ with only $2 \times 50$
events; in practice upper and lower limits on the recoil energy of signal
events, imposed to reduce backgrounds, can increase the error. The method also
loses precision if $m_\chi$ significantly exceeds the mass of the heaviest
target nucleus used.
