  In recent decades, novel magnetism of $d$- and $f$-electron compounds has
been discussed very intensively both in experimental and theoretical research
fields of condensed matter physics. It has been recognized that those material
groups are in the same category of strongly correlated electron systems, while
the low-energy physics of $d$- and $f$-electron compounds has been separately
investigated rather in different manners. One of common features of both $d$-
and $f$-electron systems is certainly the existence of active orbital degree of
freedom, but in $f$-electron materials, due to the strong spin-orbit
interaction in rare-earth and actinide ions, the physics seems to be quite
different from that of $d$-electron systems. In general, when the number of
internal degrees of freedom and relevant interactions is increased, it is
possible to obtain rich phase diagram including large varieties of magnetic
phases by using several kinds of theoretical techniques. However, we should not
be simply satisfied with the reproduction of rich phase diagram. It is believed
that more essential point is to seek for a simple principle penetrating
complicated phenomena in common with $d$- and $f$-electron materials, which
opens the door to a new stage in orbital physics. In this sense, it is
considered to be an important task of this article to explain common features
of magnetism in $d$- and $f$-electron systems from a microscopic viewpoint,
using a key concept of orbital ordering, in addition to the review of the
complex phase diagram of each material group.
