  We present a general approach to understanding the quantum phases and phase
transitions of quantum antiferromagnets in two spatial dimensions. We begin
with the simplest spin liquid state, the Z_2 spin liquid, whose elementary
excitations are spinons and visons, carrying Z_2 electric and magnetic charges
respectively. Their dynamics are expressed in terms of a doubled U(1)
Chern-Simons theory, which correctly captures the `topological' order of the
Z_2 spin liquid state. We show that the same theory also yields a description
of the variety of ordered phases obtained when one or more of the elementary
excitations condense. Field theories for the transitions and multicritical
points between these phases are obtained. We also survey experimental results
on antiferromagnets on the anisotropic triangular lattice, and make connections
between their phase diagrams and our results.
