  We review experiments on single electron transport through single quantum
dots in the presence of a microwave signal. In the case of a small dot with
well-resolved discrete energy states, the applied high-frequency signal allows
for inelastic tunnel events that involve the exchange of photons with the
microwave field. These photon assisted tunneling (PAT) processes give rise to
sideband resonances in addition to the main resonance. Photon absorption can
also lead to tunneling via excited states instead of tunneling via the ground
state of the quantum dot.
