[en] The author relates effective forces in nuclear matter and in nuclei back to the interaction between two isolated nucleons. Low-brow meson theory is used to derive the nucleon-nucleon interaction, with dispersion theoretical calculations as a guide, and a certain amount of phenomenology to pin down the parameters. The nucleon-nucleon interaction in free space is used as a starting point to discuss effective forces in nuclear matter and in nuclei. This discussion is carried out within the framework of the Landau Fermi liquid theory, initially along the lines begun by Migdal although the philosophy is somewhat different in that the author explicitly calculates as far as possible, Fermi liquid parameters from the nucleon-nucleon interaction. Certain crucial nuclear phenomena are used as a guide in this calculation, which cannot really be carried out from first principles, but must be continuously monitored by comparison of calculated results with empirical data. Information gained about effective nuclear interactions makes it possible to discuss the problem of the binding energy of nuclear matter, although this latter quantity is not easily calculated within the framework of Fermi liquid theory. It is shown that the problem of pion-nucleus interactions can be discussed within the same framework, interaction constants being related to those in the nuclear problem through the assumption of a constituent quark structure of nucleons. Finally, the problem of nuclear matter and neutron matter at high densities - the dense matter problem - is discussed, and implications of the parameters and mechanisms understood in the neutron and nuclear-matter problems are indicated. (Auth.)