phenomenon of macroscopic quantum tunneling (MQT) has
attracted attention of experimentalists and theorists
for many years. The most extraordinary feature of it is
that a macroscopic object with many degrees of freedom
manifests nonclassical behavior calculable only using
quantum mechanics. MQT has been observed to play a role
in a number of physical processes, e.g., in superfluids
and in superconductors, as well as in magnetic systems.
There are very few undisputed examples of macroscopic quantum tunneling up to date. This is one of the main reasons for a large on-going experimental effort in this field. We have investigated MQT in relation with nucleation problems in solid and superfluid helium. Contrary to the common problems caused by dissipation in MQT, the helium experiments suffer easily from heterogeneity due to confining walls. Hence, extreme care has to be exercized to eliminate spurious effects and to reveal features generic to homogeneous nucleation.