  Novel simple properties of the monopole component of the effective
nucleon-nucleon interaction are presented, leading to the so-called
monopole-based universal interaction. Shell structures are shown to change as
functions of $N$ and $Z$ consistently with experiments. Some key cases of this
shell evolution are discussed, clarifying the effects of central and tensor
forces. The validity of the present tensor force is examined in terms of the
low-momentum interaction V$_{low k}$ and the Q$_{box}$ formalism.
