  Kilohertz QPO sources are reasonably well-characterized observationally, but
many questions remain about the theoretical framework for these sources and the
consequent implications of the observations for disk physics, strong gravity,
and dense matter. We contrast the predictions and implications of the most
extensively studied class of kilohertz QPO models, the beat-frequency models,
with those of alternative classes of models. We also discuss the expected
impact of new observations of these sources with satellites such as Chandra,
XMM, and Constellation-X.
