Publisher Summary Skutterudite is the name of a CoAs3-based mineral that was first extensively mined as a source of cobalt and nickel. This chapter discusses the structural, electronic, and magnetic properties of the stoichiometric lanthanide skutterudite compounds of the form RM4X12. The low temperature properties will be of particular interest, since these compounds exhibit a rich variety of electronic and magnetic ground states that include: heavy-fermion mediated superconductivity, ferromagnetism, antiferromagnetism, hybridization gaps (Kondo insulator behavior), nonfermi-liquid behavior and quantum critical points, quadrupolar ordering, and field-induced heavy-fermion states. Several Ce-filled skutterudites are narrow gap semiconductors (also called Kondo insulators), where the gap is created by a strong hybridization between the Ce 4f level, and the transition metal and pnicogen states near the fermi energy. The Nd, Eu, Gd, and Tb filled skutterudites order magnetically at temperatures ranging from 2K for NdFe4P12 to 100K for EuFe4P12. Coupling between the lanthanide magnetic moments and the conduction electrons is large in these compounds. This large coupling results in unusual peaks in electrical transport data near the onset of magnetic ordering, and in some cases multiple magnetic transitions.