The discovery of a large number of shallow and deep levels in ultra-pure germanium has led to a renewed interest in this elemental
semiconductor. The new levels are created by complexes typically consisting of one substitutional impurity such as silicon,
oxygen, carbon, beryllium, zinc, copper, etc. and one or more interstitial impurities such as hydrogen or lithium. The experimental
techniques and results which have been used to determine the structure and composition of these centers are reviewed. One
of the most intriguing features of the acceptor centers, an electronic ground-state which does not split under uniaxial compression,
has been explained with a tunneling impurity model.
Double acceptors, helium analoga in semiconductors, have been investigated recently with IR spectra and photoluminescence.
The new results indicate that contrary to earlier assumptions all double acceptors exhibit split ground-states, a consequence
of the hole-hole interaction.