Invited review abstract

We present a summary of the current sample of young stars and brown dwarfs with direct and accurate mass measurements, and we summarize the extent to which the masses predicted by current theoretical pre-main-sequence evolution models for these objects agree with the empirical determinations. We highlight a few recent discoveries among the sample of young eclipsing binaries that may have particularly important implications for the nature and origin of the initial mass function at very low masses, including: (1) the effects of magnetic activity on the inferred masses of very low mass stars and brown dwarfs, (2) the protostellar collapse and accretion histories of close binaries, and (3) the frequency of high-order (e.g. triple) systems.