The paper by J.G. Bednorz and K.A. Müller reports on the synthesis and properties of metallic, oxygen-deficient compounds in the Ba – La – Cu – O system, specifically focusing on compositions of the form Ba_xLa_{1-x}Cu_yO_{3-(y)}. These samples, prepared through coprecipitation and sintering, exhibit a complex phase structure with three distinct phases, one of which is a perovskite-like mixed-valent copper compound. Upon cooling, the samples show a linear decrease in resistivity followed by an approximately logarithmic increase, indicating the onset of localization. However, an abrupt decrease in resistivity by up to three orders of magnitude is observed, suggesting the potential for percolative superconductivity. The highest onset temperature is around 30 K, which is significantly reduced by high current densities. The authors suggest that this behavior may be due to both the percolative nature of the material and the presence of 2D superconducting fluctuations in the double perovskite layers. The study highlights the potential for high-$T_c$ superconductivity in perovskite-type or related metallic oxides, particularly those with strong electron-phonon interactions and high carrier densities.The paper by J.G. Bednorz and K.A. Müller reports on the synthesis and properties of metallic, oxygen-deficient compounds in the Ba – La – Cu – O system, specifically focusing on compositions of the form Ba_xLa_{1-x}Cu_yO_{3-(y)}. These samples, prepared through coprecipitation and sintering, exhibit a complex phase structure with three distinct phases, one of which is a perovskite-like mixed-valent copper compound. Upon cooling, the samples show a linear decrease in resistivity followed by an approximately logarithmic increase, indicating the onset of localization. However, an abrupt decrease in resistivity by up to three orders of magnitude is observed, suggesting the potential for percolative superconductivity. The highest onset temperature is around 30 K, which is significantly reduced by high current densities. The authors suggest that this behavior may be due to both the percolative nature of the material and the presence of 2D superconducting fluctuations in the double perovskite layers. The study highlights the potential for high-$T_c$ superconductivity in perovskite-type or related metallic oxides, particularly those with strong electron-phonon interactions and high carrier densities.