The filled skutterudite compounds, which have the formula MT₄X₁₂, where M = alkali metal, alkaline earth, lanthanide, actinide, T = Fe, Ru, Os, and X = P, As, Sb, have attracted a great deal of interest in recent years because they exhibit a wide range of correlated electron phenomena and are promising candidates for thermoelectric applications. Correlated electron phenomena that have been observed include superconductivity, magnetic order, quadrupolar order, valence fluctuations, heavy fermion behavior, non-Fermi liquid behavior, and metal-insulator transitions. In this talk, we describe recent experiments on the Pr-based filled skutterudites, particularly PrOs₄Sb₁₂, Pr(Os_1-xRu_x)₄Sb₁₂, and PrOs₄As₁₂, whose ground states are determined by a delicate interplay between hybridization of Pr³⁺ localized 4f- and itinerant-electron states, crystalline electric field splitting of the Pr³⁺ energy levels, magnetic and quadrupolar interactions, and electronic band stricture. The compound PrOs₄Sb₁₂ is of considerable interest for several reasons: (1) it is the first example of a heavy fermion superconductor based on Pr (all of the other known heavy fermion superconductors are compounds of Ce or U); (2) it exhibits a type of unconventional strong coupling superconductivity that breaks time reversal symmetry, apparently consists of several distinct superconducting phases, some of which appear to have point nodes in the energy gap, and may involve triplet spin pairing of electrons; (3) there is a high field ordered phase between 4.5 T and 16 T and below ~1 K that has been identified with antiferroquadrupolar order, indicating that the superconductivity occurs in the proximity of a quadrupolar quantum critical point; and (4) the pairing of superconducting electrons may be mediated by electric quadrupole fluctuations, rather than magnetic dipole fluctuations. In the Pr(Os_1-xRu_x)₄Sb₁₂ pseudoternary system, increasing the Ru concentration x results in a monotonic increase in the splitting between the Pr³⁺ singlet ground state and triplet first excited state from ~7 K at x = 0 to 60 K at x = 1, a minimum in the T_c vs x curve at x ~ 0.6, and an apparent change in the nature of the superconductivity from unconventional to conventional BCS at x ~ 0.3. In contrast, the compound PrOs₄As₁₂ undergoes transitions at 2.3 K and 2.2 K in zero-field into two ordered phases that can be suppressed to 0 K with magnetic fields of 2 T and 3.2 T. The low field ordered phase is antiferromagnetic, while the nature of the higher field ordered phase has not yet been determined. The temperature and field dependences of the specific heat and electrical resistivity indicate that PrOs₄As₁₂ is a Kondo lattice system with a small Kondo temperature T_K ~ 1 K and an enormous electronic specific heat coefficient of ~1 J/mol K².