  We briefly review the oscillator and spin bath models of quantum
environments, which can be used to describe the low-energy dynamics of open
quantum systems. We then use them to discuss both the mechanisms causing
decoherence at low $T$, and the dynamics of this decoherence. This is done
first for a central 2-level system coupled to these environments- the results
can be applied to the dynamics of quantum nanomagnets and superconducting
SQUIDs, where large-scale tunneling of magnetisation and flux take place.
Decoherence in these systems is caused principally by coupling to electrons and
nuclear spins, the spin bath couplings are particularly dangerous at low $T$.
  These models are also generalised to discuss quantum measurements in which
the measured system, the measuring apparatus, and the environment are treated
quantum mechanically. The results can be used to calculate the dynamics of
coupled SQUIDs and/or nanomagnets, in which one acts as a measuring apparatus
and the other exhibits large-scale quantum superpositions. The same model can
be used to describe coupled qubits.
