  We consider the problem of estimating the virial mass of a dark halo from the
positions and velocities of a tracer population. Although a number of general
tools are available, more progress can be made if we are able to specify the
functional form of the halo potential (although not its normalization). Here,
we consider the particular case of the cosmologically motivated
Navarro-Frenk-White (NFW) halo and develop two simple estimators. We
demonstrate their effectiveness against numerical simulations and use them to
provide new mass estimates of Carina, Fornax, Sculptor, and Sextans dSphs.
