  Extensive experimental investigations of the magnetic structures and
excitations in the XY pyrochlores have been carried out over the last decade.
Three families of XY pyrochlores have emerged: Yb$_2B_2$O$_7$, Er$_2B_2$O$_7$,
and, most recently, $AA'$Co$_2$F$_7$. In each case, the magnetic cation (either
Yb, Er, or Co) exhibits XY anisotropy within the local pyrochlore coordinates,
a consequence of crystal field effects. Materials in these families display
rich phase behavior and are candidates for exotic ground states, such as
quantum spin ice, and exotic ground state selection via order-by-disorder
mechanisms. In this review, we present an experimental summary of the ground
state properties of the XY pyrochlores, including evidence that they are
strongly influenced by phase competition. We empirically demonstrate the
signatures for phase competition in a frustrated magnet: multiple heat capacity
anomalies, suppressed $T_N$ or $T_C$, sample and pressure dependent ground
states, and unconventional spin dynamics.
