Knowledge of the atomic hydrogen density proÿle (H) in the upper atmosphere is important both for studies of mesospheric-lower thermospheric (MLT) chemistry and for determining upper thermospheric-lower exospheric densities needed for realistic modeling of geocoronal density and ballistic ux distributions. Early work is reviewed and the sparseness of relevant data on the variation of [H] with altitude is emphasized. While optical measurements (e.g., of Lyman ) constitute the most practical approach currently available, data analysis requires radiative transport (RT) modeling and H retrieval is e ectively limited to obtaining values for a small number of modeling parameters. The key physical parameters of interest are the exobase density [H] c , the photochemically initiated upward ux , and the density at the mesospheric peak [H] peak . This paper discusses RT-modeling analysis of dayside Lyman disk-to-limb scanning intensity (4 I ) measurements couched in terms of these parameters, aided by the use of a physically motivated parametric model for thermospheric H overcoming MSIS limitations at MLT altitudes. Illustrative sensitivity study results under solar maximum conditions are presented. The mesospheric density peak directly impacts 4 I proÿles over a wide range of dayside solar zenith angles under such conditions. While mild di culties in unraveling [H] peak and in retrieval e orts may arise, analysis of dayside Lyman 4 I disk-tolimb scanning data expected from several future satellite FUV instruments should permit determination of [H] c and over much of the dayside.