  Strongly correlated materials are characterized by the presence of
electron-electron interactions in their electronic structure. They often have
remarkable properties and transitions between competing phases of very
different electronic and magnetic order. This thesis focuses on strongly
correlated $f$-electron compounds containing Ce, Sm, and U. These materials
exhibit a so-called heavy-fermion or Kondo-lattice behavior. They can become
insulating due to hybridization effects (Kondo-insulator) or develop multipolar
(hidden) order. Kondo insulators have recently been discussed in the context of
strongly correlated topological insulators. This new aspect caused an enormous
activity in the field of Kondo insulators, theoretically as well as
experimentally. Multipolar order as well as the formation of a Kondo insulating
state strongly depend on the symmetry of the $f$ states involved. Also the
character of the surface states in a topological insulator is determined by the
properties of the bulk states. Therefore the scope of this thesis has been to
unveil the underlying symmetries of the bulk $f$ states. Here the compounds
CeB$_6$, UO$_2$, and URu$_2$Si$_2$, which exhibit multipolar order, as well as
the Kondo insulators (semimetals) SmB$_6$ and CeRu$_4$Sn$_6$ have been studied.
