YBa2Cu3Oy thin films have been deposited in-situ on several substrate materials using pulsed excimer laser deposition. On the substrates, SrTiO3, MgO, LaA103, and yttrium-stabilized zirconia (YSZ), excellent films were obtained. These films had high superconducting transition temperatures (91K) with narrow transition widths (0.5K), metallic conductivity in the normal state, low room-temperature resistivity (250 Ω-cm), high critical currents (3x107 A/cm2 at 4.2K), c-axis orientation perpendicular to the plane of the film, and epitaxial alignment with the substrate. On the more technologically relevant substrates of A12O3 and Si, less optimal results were obtained. The transition temperatures were high (86-88K) and metallic conductivity way obtained in the normal state. However, the room-temperature and microwave surface resistivities were higher and the critical currents were lower than for the above benchmark substrates. These diminished transport properties correlate with the imperfect alignment and epitaxy of the YBCO and substrate. For A12O3 substrates, a narrow substrate-temperature window was found for the best in-situ YBCO films. The poorer transport properties correlate with the lack of registry of the YBCO a-b plane with the sapphire r-plane. For Si substrates, a buffer layer is required due to high reactivity even at substrate temperatures as low as 550 C. YSZ provides a good buffer, and our best results were obtained on clean, hydrogen-terminated surfaces rather than oxidized Si. The amount of Y2O3 in ZrO2 was varied, and the best films were obtained with x near 0.1 where (ZrO2)1-x(Y2O3)x is cubic. Epitaxial alignment of the YBCO with the Si was achieved, but there was a substantial spread in orientations, accounting for the diminished transport properties. © 1990 SPIE.