The dynamical structure factor S (k, w) and elementary excitations in normal liquid He' are studied by employing the memory function formalism. Canonical correlation functions and response functions for atomic number den:oity and spin density fluctuations are treated in a self-consistent manner to express the response functions in terms of the Lindhard function (the response function of an ideal Fermi gas) and effective potentials. Exact formal expressions for the effective potentials are composed of two parts, static and dynamic, the former being a quanta! version of the Percus effective potential, while the latter originating from an interaction-part of second order memory functions (ISMF). A discussion is given on the interrelation of the effective potentials in small wavenumber and small frequency regions to those obtained by solving transport equations for quasi-particles. In the evaluation of S (k, w) a spectral function So (k, w) for the atomic number density fluctuations yielding zero sound modes is calculated by taking the ISMF to be Gaussian, while that for the spin density fluctuations 5 1 (k, w) giving paramagnon modes is calculated by using the static effective potential. The dynamical structure factor so obtained is compared with the experi mental data of Skold et al. Fairly good agreement with the experimental result is obtained for 0.9A-'<k<2.oA-•. In the case of S 0 (k,w) inclusion of the dynamic nature of the effec tive potential is shown to be essential, which shows the breakdown of the RP A as in the case of atomic dynamics in classical simple liquids. Considerable theoretical progress has been made m our understanding of the physical properties of Fermi liquids such as liquid He 3, electron liquids and nuclear matters as well. Almost all theories here, in which any discussion conventionally starts with the ideas and concepts originally developed by Landa u/l are generally considered to be valid for low frequency (compared with frequency corresponding to the Fermi energy EF) and long wavelength (compared with inter-atomic spacing) excitations. Recently, the response of strongly coupled Fermi systems at a micro scopic level to an external probe with momentum transfer Ptk and energy transfer Ptw comparable with the Fermi momentum PF and EF, respectively, has been studied for normal liquid He 3 2l,sJ and simple metals 4l, 5l by using inelastic neutron and X-ray and/or electron scattering measurements, respectively. A wealth of information on dynamics of particles in Fermi liquids 6l outside the region of the applicability of the Landau Fermi liquid theory!l has been thereby obtained. For example, Shild