Operating at CERN since 1991 is a 3-tonne liquid argon time projection chamber, a detector breakthrough which combines the visual advantages of bubble chamber tracks with the flexibility of fully electronic data acquisition. The 3-tonne chamber is a prototype for a much larger configuration for the ICARUS* solar neutrino and proton decay detector to be installed in the Italian Gran Sasso underground laboratory. ICARUS (Imaging Cosmic And Rare Underground Signals) is built around the cryogenic imaging chamber idea initially proposed by Carlo Rubbia in 1977. With electrons drifting for a relatively long time (several milliseconds) and with sensitive amplifiers picking up the ionization from just a few millimetres of track, events can be imaged inside the cryogenic volume. A special arrangement of readout wires provides drift time measurements and ensures simultaneous imaging in several different views. The prototype has shown that the challenges of obtaining ultra-pure argon and operating readout techniques for large sensitive volumes have been met. The full ICARUS detector (with three liquid argon modules each containing 5,000 tonnes) will be able to detect low energy electrons (down to a few MeV) emerging from solar neutrino interactions, proton decays, or other rare events over a large volume.