  We discuss a possible origin of the recently discovered microwave-induced
zero-resistance states in very-high-electron-mobility two-dimensional electron
systems. We suggest a scenario, in which two mechanisms, bulk and edge,
contribute to the measured photosignal. Zero-resistance states are assumed to
be due to a drift plasma instability, developing {\em near the edge} of the
system under the microwave radiation. The proposed scheme qualitatively agrees
with the microwave power, temperature, frequency, magnetic field, and mobility
dependencies of the measured photosignal.
