  We present new infrared photometry for 61 halo and disk stars around the
stellar/substellar boundary. These data are combined with available optical
photometry and astrometric data to produce color--color and absolute
magnitude--color diagrams. The disk and halo sequences are compared to the
predictions of the latest model atmospheres and structural models.
  We find good agreement between observation and theory except for known
problems in the V and H passbands probably due to incomplete molecular data for
TiO, metal hydrides and H$_2$O. The metal--poor M subdwarfs are well matched by
the models as oxide opacity sources are less important in this case. The known
extreme M subdwarfs have metallicities about one--hundredth solar, and the
coolest subdwarfs have T$_{eff}\sim 3000$ K with masses $\sim
$0.09M/M$_{\odot}$. The grainless models are not able to reproduce the flux
distributions of disk objects with T$_{eff} <$ 2500 K, however a preliminary
version of the NextGen--Dusty models which includes homogeneous formation and
extinction by dust grains {\it is} able to match the colors of these very cool
objects. The least luminous objects in this sample are GD165B, three DENIS
objects --- DBD0205, DBD1058 and DBD1228 --- and Kelu-1. These have
T$_{eff}\sim$ 2000 K and are at or below the stellar limit with masses
$\leq$0.075M/M$_{\odot}$. Photometry alone cannot constrain these parameters
further as the age is unknown, but published lithium detections for two of
these objects (Kelu-1 and DBD1228) imply that they are young (aged about 1 Gyr)
and substellar (mass $\leq$0.06M/M$_{\odot}$).
