The photochemistry and transport of minor neutral constituents in the mesosphere and lower thermosphere are reviewed. The assumptions and inherent weaknesses of current theoretical models are discussed in detail. A better theoretical description of turbulent transport processes is needed for future models. The effects of planetary-scale circulation systems on minor constituents should be investigated. Quenching of N(2D) by O2 is the most likely source of the large NO concentrations present in the mesosphere, though laboratory studies are needed to determine the actual sources of N(2D) under thermospheric conditions. Recent mass spectrometric measurements of the ratio [O]/[O2] at 120 km calls for considerable rethinking on the oxygen problem. A critical review of theoretical models of a hydrogen/oxygen atmosphere is given and problems associated with boundary conditions are pointed out in various models. The need for more measurements of hydrogen constituents is emphasized.