Colossal magnetoresistance (CMR) effects have been observed in mixed-valent perovskite manganites . The mixed-valent manganites can be prepared by the partial substitution of La3+ by various divalent ions such as Pb, Ca, Sr or Ba or cation deficiency in perovskite LaMn03. Other rare earth ions with relatively smaller ionic size (e.g., Pr, Nd or Sm) also form in the perovskite structure, and divalent substitution or cation deficiency in the perovskites with these rare earths can also induce CMR. It turns out that (La,Ca)Mn03 is the prototype of mixed-valent perovskite manganites in the following sense. The Ca ionic size is almost identical to the La ionic size, and thus a true solid solution forms in the entire range of Ca concentrations. Furthermore, the structure, unlike other perovskite manganites, remains orthorhombic below ~700 K in the entire doping concentration. Therefore, the (La,Ca)MnC>3 system is emphasized in this chapter to explore the carrier-concentration dependence of the physical properties in perovskite manganites.