The critical current density across grain boundaries of twinned thin-film Bi2Sr2CaCu208+, bicrystals has been measured as a function of the tilt angle O. For 0'=0 to 45, the ratio of intergrain to intragrain critical current density decreases exponentially with increasing tilt angle. Surprisingly, this orientation dependence is very similar to that observed for YBa2Cu30& bicrystals. Microstructural investigations of plan view samples show a wavy grain boundary of the superconductor with a roughness of 100 nm to 1 pm which does not originate from the roughness of the substrate grain boundary (1 -3 nm) but is caused by an island-plus-layer growth of twin domains. In general, one of the adjacent grains has a low-indexed habit plane at the boundary.Whereas the critical current density, j,, of thin films of high-T, superconductors (HTS's) is already high enough for numerous applications, polycrystalline materials, which are required for making wires or tapes, are characterized by rather low current densities. A variety of effects is responsible for the low j, of polycrystalline HTS's. However, j, in polycrystals is lowered by voids, cracks, and second phases. Furthermore, in nonoriented grains, the current has to flow along the c direction, which has a low j, . Finally, grain boundaries (GB's) reduce j,, in contrast to certain low T, superconductors, in which GB s enhance j, by acting as pinning sites. The reason is that in HTS's the extremely short coherence length of about (, b= 1 nm causes poor coupling of the superconducting order parameter across grain boundaries. Therefore, HTS-GB's act as weak links. This behavior has been utilized to fabricate Josephson junctions of high quality by using thin film bicrystals of either YBa2Cu307 (YBCO) or BizSr2CaCu20s+"(BSCCO). ' Unfortunately, the short coherence length is responsible for the sensitivity of the electrical properties on small defects, too. Thus, it is difficult to correlate the macroscopic properties to microstructural features and vice versa. By measuring the superconducting transport properties of individual grain boundaries and their adjacent grains in YBCO thin film bicrystals, Dimos et al. demonstrated that the critical current across a GB, j, , is quite sensitive to the misorientation angle 8. Since then, emphasis was put on the investigation of transport properties of GB's but the effort was mainly restricted to YBCO due to the high quality of thin films. 'In the following, some properties of single, artificial GB's in high-quality thin film bicrystals of BSCCO will be presented. The measurement of the critical current density as a function of the misorientation angle in conjunction with high-resolution transmission electron microscopy gives new information on the superconducting transport properties of GB's. Additionally, the film growth across the substrate GB will be presented schematically.BSCCO films, about 200 to 400 nm thick, were deposited by excimer laser ablation on commercially available SrTi03 bicrystals. Bicrystalline substrates with variou...