  Double layer quantum Hall systems have interesting properties associated with
interlayer correlations. At $\nu =1/m$ where $m$ is an odd integer they exhibit
spontaneous symmetry breaking equivalent to that of spin $1/2$ easy-plane
ferromagnets, with the layer degree of freedom playing the role of spin. We
explore the rich variety of quantum and finite temperature phase transitions in
these systems. In particular, we show that a magnetic field oriented parallel
to the layers induces a highly collective commensurate-incommensurate phase
transition in the magnetic order.
