  Mass is the fundamental parameter that governs the evolution of stars, brown
dwarfs, and gas-giant planets. Thus, direct mass measurements are essential to
test the evolutionary and atmospheric models that underpin studies of these
objects. We present results from our program to test models using precise
dynamical masses for visual binaries based on Keck laser guide star adaptive
optics astrometric monitoring of a sample of over 30 ultracool (> M6) objects
since 2005. In just the last 2 years, we have more than tripled the number of
late-M, L, and T dwarf binaries with precise dynamical masses. For most field
binaries, based on direct measurements of their luminosities and total masses,
we find a "temperature problem" in that evolutionary model radii give effective
temperatures that are inconsistent with those from model atmosphere fitting of
observed spectra by 100-300 K. We also find a "luminosity problem" for the only
binary with an independent age determination (from its solar-type primary via
age-activity-rotation relations). Evolutionary models underpredict the
luminosities of HD130948BC by a factor of ~2, implying that model-based
substellar mass determinations (e.g., for directly imaged planets and cluster
IMFs) may be systematically overestimating masses. Finally, we have employed
the current sample of binary orbits to carry out a novel test of the earliest
evolutionary stages, by using the distribution of orbital eccentricities to
distinguish between competing models of brown dwarf formation.
