"You cannot improve what you cannot measure". "What gets measured, gets done." These truisms about quality improvement reflect the importance of performance measurement which is being used not only for P4P but also for network selection, reorganizing care delivery processes, and transforming the culture of health care organizations. For all of the policy interest in core data sets and standardization of measures, the current context offers anything but. The varieties of measures and measurers seems to be proliferating rather than consolidating. There are controversies and quandaries in performance measurement, as well as lurking legal liabilities both for those who would select and use measures as well as those who do not or do so ineffectively. In "The Performance Measures Ball: Too Many Tunes, Too Many Dancers?" Alice Gosfield offers a view of the new Medicare Modernization Act initiatives which relate to measurement; explores why the explosion of interest in measurement and measures now; clarifies concepts around measurement; provides a snapshot of the major players and their approaches including CMS, NQF, JCAHO, AHRQ and IHI; considers the significant controversies surrounding measurement initiatives; and speculates on legal pitfalls in this essential component of quality improvement and a business case for quality.