  The tau sneutrino is proposed as a candidate for galactic halo dark matter,
and as the cold dark matter (CDM) component of the universe. A
lepton-number-violating sneutrino mass, \tilde{\nu}\tilde{\nu}, splits the tau
sneutrino into two mass eigenstates: \tilde{\nu} \rightarrow \tilde{\nu}_\pm.
The absence of a Z \tilde{\nu}_- \tilde{\nu}_- coupling implies that the
lighter mass eignestate, \tilde{\nu}_-, does not annihilate via the s-channel
Z-exchange to a low cosmological abundance, and furthermore, halo sneutrinos do
not scatter excessively in Ge detectors. For the majority of the relevant
parameter space, the event rate in Ge detectors is \geq 10^{-2} events/kg/day.
The lepton number violation required for sneutrino CDM implies that the tau
neutrino mass is m_{\nu_\tau} \gtrsim 5 MeV, large enough to be excluded by B
factory experiments. Events of the form l^+ l^- \not E or jj \not E , with low
m_{ll} or m_{jj}, may be observed at LEP2. A seesaw mechanism is investigated
as the origin for the lepton number violation, and several other cosmological
and particle physics consequences of sneutrino CDM are discussed.
