  We show that physics of multi-GEM structures is rather complex, regarding the
number of phenomena affecting detector performance. The high-pressure operation
in noble gases and the ion feedback are considered in more detail. It is
proposed that the dominant avalanche mechanism in He and Ne, at high pressures,
is the associative ionization. Ion feedback effects related to the dependence
on gas, pressure and gain and to possible avalanche extension outside the GEM
holes are discussed.
