  Often, one would like to determine some observable A, but can only measure
some (hopefully related) observable M. This can arise, for example, in quantum
eavesdropping, or when the research lab budget isn't large enough for that 100%
efficient photodetector. It also arises whenever one tries to jointly determine
two complementary observables via some measurement M.
  This raises three natural questions: (i) What is the best possible estimate
of A from M ? ; (ii) How 'noisy' is such an estimate ? ; and (iii) Are there
any universally valid uncertainty relations for joint estimates ? Quite general
answers, and applications to heterodyne detection and EPR joint measurements,
are briefly reviewed.
