Direct observations and measurements of spatial magnetic flux distribu
tions on the surfaces of YBaCuO superconducting thin films and thin si
ngle crystals realized under application of external magnetic field an
d transport current are accomplished using magneto-optical indicators.
These investigations are compared with theoretical calculations yield
ing critical current densities. Special attention is paid to the influ
ence of defects on the magnetic properties. Furthermore, specific feat
ures of flux penetration into flat superconductors are revealed.