  We present a general introduction to the non-zero temperature dynamic and
transport properties of low-dimensional systems near a quantum phase
transition. Basic results are reviewed in the context of experiments on the
spin-ladder compounds, insulating two-dimensional antiferromagnets, and
double-layer quantum Hall systems. Recent large N computations on an extended
t-J model (cond-mat/9906104) motivate a global scenario of the quantum phases
and transitions in the high temperature superconductors, and connections are
made to numerous experiments.
