In this paper we discuss some (and only a small fraction) of the interesting properties of superfluid3Heat the low-temperature limit. We concentrate on the unique behavior and applications of the very dilute excitation gas at the lowest temperatures. This gas has been used for among other things, the probing of theA–Bphase interface, the detection of low-energy particle events and in the simulation of the creation of cosmic strings. ©1997 American Institute of Physics.