The development of three directions in nuclear physics, originating from three ideas of Niels Bohr, is analyzed: 1) the compound nucleus: is the compound state entirely chemical? what does the energy distribution of neutron resonances imply? ``dynamic intensification'' of weak effects; the role of fluctuations and the description of the kinetics; and, pre-equilibrium processes; 2) collective motions: how the collective and single-particle degrees of freedom coexist with one another; parallel formation of the shell model and the theory of collective oscillations; the generalized model; the problem of the moment of inertia; pair correlations and ``superfluidity'' in the nucleus; giant resonances; and theory of finite Fermi systems; and, 3) the fission process: fission in the liquid drop model and in the generalized model; shell corrections; double-hump fission barriers; fission isomers; nonconservation of parity in fission; and, ``exotic'' asymmetric fission. The emphasis is on the elucidation of the development of physical ideas, so that computational details are omitted.