  The uncertainty principle being a cornerstone of quantum mechanics, it is
surprising that in nearly 90 years there have been no direct tests of
measurement uncertainty relations. This lacuna was due to the absence of two
essential ingredients: appropriate measures of measurement error (and
disturbance), and precise formulations of such relations that are {\em
universally valid}and {\em directly testable}. We formulate two distinct forms
of direct tests, based on different measures of error. We present a prototype
protocol for a direct test of measurement uncertainty relations in terms of
{\em value deviation errors} (hitherto considered nonfeasible), highlighting
the lack of universality of these relations. This shows that the formulation of
universal, directly testable measurement uncertainty relations for {\em
state-dependent} error measures remains an important open problem. Recent
experiments that were claimed to constitute invalidations of Heisenberg's
error-disturbance relation are shown to conform with the spirit of Heisenberg's
principle if interpreted as direct tests of measurement uncertainty relations
for error measures that quantify {\em distances between observables}.
