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abstract: 'We present a sequence of high resolution (R$\sim$20,000 or 15 km s$^{-1}$) infrared spectra of stars and brown dwarfs spanning spectral types M2.5 to T6. Observations of 16 objects were obtained using eight echelle orders to cover part of the $J$-band from 1.165-1.323 $\mu$m with NIRSPEC on the Keck II telescope. By comparing opacity plots and line lists, over 200 weak features in the $J$-band are identified with either FeH or H$_{2}$O transitions. Absorption by FeH attains maximum strength in the mid-L dwarfs, while H$_{2}$O absorption becomes systematically stronger towards later spectral types. Narrow resolved features broaden markedly after the M to L transition. Our high resolution spectra also reveal that the disappearance of neutral Al lines at the boundary between M and L dwarfs is remarkably abrupt, presumably because of the formation of grains. Neutral Fe lines can be traced to mid-L dwarfs before Fe is removed by condensation. The neutral potassium (K I) doublets that dominate the $J$-band have pressure broadened wings that continue to broaden from $\sim$ 50 km s$^{-1}$ (FWHM) at mid-M to $\sim$ 500 km s$^{-1}$ at mid-T. In contrast however, the measured pseudo-equivalent widths of these same lines reach a maximum in 