With a mean- or average-field picture we mean that a given nucleon (either a proton or a neutron) moves in an average potential field which is set up by all other nucleons in the system. Consider for example a nucleus like \( {}^{17}\mbox{O} \) with nine neutrons and eight protons. Many properties of this nucleus can be interpreted in terms of a picture where we can view it as one neutron on top of \( {}^{16}\mbox{O} \). We infer from data and our theoretical interpretations that this additional neutron behaves almost as an individual neutron which sees an average interaction set up by the remaining 16 nucleons in \( {}^{16}\mbox{O} \). A nucleus like \( {}^{16}\mbox{O} \) is an example of what we in this course will denote as a good closed-shell nucleus. We will come back to what this means later.