  A Time Projection Chamber (TPC) is a powerful detector for 3-dimensional
tracking and particle identification for ultra-high multiplicity events. It is
the central tracking device of many experiments, e.g. the ALICE experiment at
CERN. The necessity of a switching electrostatic gate, which prevents ions
produced in the amplification region o MWPCs from entering the drift volume,
however, restricts its application to trigger rates of the order of 1 kHz.
  Charge amplification by Gas Electron Multiplier (GEM) foils instead of
proportional wires offers an intrinsic suppression of the ion backflow,
although not to the same level as a gating grid. Detailed Monte Carlo
simulations have shown that the distortions due to residual space charge from
back-drifting ions can be limited to a few cm, and thus can be corrected using
standard calibration techniques. A prototype GEM-TPC has been built with the
largest active volume to date for a detector of this type. It has been
commissioned with cosmics and particle beams at the FOPI experiment at GSI, and
was employed for a physics measurement with pion beams.
  For future operation of the ALICE TPC at the CERN LHC beyond 2019, where
Pb-Pb collision rates of 50 kHz are expected, it is planned to replace the
existing MWPCs by GEM detectors, operated in a continuous, triggerless readout
mode, thus allowing an increase in event rate by a factor of 100. As a first
step of the R&D program, a prototype of an Inner Readout Chamber was equipped
with large-size GEM foils and exposed to beams of protons, pions and electrons
from the CERN PS.
  In this paper, new results are shown concerning ion backflow, spatial and
momentum resolution of the FOPI GEM-TPC, detector calibration, and dE/dx
resolution with both detector prototypes. The perspectives of a GEM-TPC for
ALICE with continuous readout will be discussed.
