  We review four mechanisms for forming brown dwarfs: (i) turbulent
fragmentation (producing very low-mass prestellar cores); (ii) gravitational
instabilities in discs; (iii) dynamical ejection of stellar embryos from their
placental cores; and (iv) photo-erosion of pre-existing cores in HII regions.
We argue (a) that these are simply the mechanisms of low-mass star formation,
and (b) that they are not mutually exclusive. If, as seems possible, all four
mechanisms operate in nature, their relative importance may eventually be
constrained by their ability to reproduce the binary statistics of brown
dwarfs, but this will require fully 3-D radiative magneto-hydrodynamic
simulations.
