Measurements of the temperature dependence of the resistivity ρ(T) and thermoelectric power α(T) under several hydrostatic pressures on l8O/l6O isotope-exchanged (La1-xNdxCa0.3MnO3 polycrystalline samples spanning the structural O'-O orthorhombic transition have demonstrated that the perovskite tolerance factor t increases with pressure, signaling an unusually compressible Mn-0 bond. They have also indicated a change at the O'-O transition from static to dynamic cooperative Jahn-Teller deformations, from a second-order to a first-order magnetic transition, from Mn(IV) to two-Mn polarons in the paramagnetic region, from Mn(IV) polarons to a vibronic state below Tc, and a phase segregation in the O phase above Tc that traps out mobile polarons into ferromagnetic Mn(IV)-rich clusters within a Mn(IV)-poor matrix. Specific heat data show a transfer of spin entropy to configurational entropy on cooling through Tcin the O phase, and a single-crystal study of La1-xSrxMnO3, x = 0.12 and 0.15, has demonstrated a transition from polaronic to itinerant e electrons below Tc within the O phase. Magnetic-susceptibility measurements in low fields confirm the phase segregation above Tc and the existence of a ferromagnetic glass below Tc in the O phase. The intrinsic “colossal” magnetoresistance (CMR) is attributed to the growth to their percolation threshold of the ferromagnetic clusters existing above Tc, and the dramatic rise in Tc with increasing tolerance factor t in the O-orthorhombic phase to an increase in the density, and also the mobility, of the untrapped polarons above Tc.