We show how to estimate the enclosed mass from the observed motions of anensemble of test particles. Traditionally, this problem has been attackedthrough virial or projected mass estimators. Here, we examine and extend thesesystematically, and show how to construct an optimal estimator for any givenassumption as to the potential. The estimators do not explicitly depend on anyproperties of the density of the test objects, which is desirable as inpractice such information is dominated by selection effects. As particularexamples, we also develop estimators tailored for the problem of estimating themass of the Hernquist or NFW dark matter haloes from the projected positionsand velocities of stars.
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