Different synthesis techniques for the preparation of dense superconducting ceramics are discussed, and a sol-gel process is shown to be very promising. The effect of oxygen content, and the effect of substitution of Ni and Zn for copper, on the structural, transport and superconducting properties of the La-Sr-Cu-O and Y-Ba-Cu-O systems are presented. We find that substitution on the copper sites destroys T//c in the La-Sr-Cu-O system and decreases it in the Y-Ba-Cu-O system, and this effect is insensitive as to whether the 3d metal is magnetic (Ni) or diamagnetic (Zn). A detailed study of the YBa//2Cu//3O//7// minus //y system as a function of oxygen content (y) shows that superconductivity can be destroyed in these materials by the removal of oxygen and restored by reinjecting oxygen; either thermally at 500 degree C or at temperatures (80 degree C) compatible with device processing by means of a novel plasma oxidation process. Of scientific interest, the plasma process induces bulk superconductivity in the undoped La//2CuO//4.