The electronic properties of high-Tc superconductors are extremely anisotropic. Consequently theories which describe their behaviour must be able to distinguish between the motion of the superconducting carriers in different directions. For , the crystal structure is orthorhombic (see Fig. 2.9). The layered crystal structure of results in a strong -anisotropy, while the orthorhombic distortions due to the Cu-O chains introduce another - anisotropy. The anisotropic nature of various physical quantities measured in single crystals of have been well documented. For instance, the electrical resistivity along the and -directions of the unit cell varies linearly with temperature, while along the -direction there is a distinct upward curvature for increasing temperature, which has been attributed to conductivity along the Cu-O chains [64,65]. Critical-field, critical-current and magnetic penetration depth measurements in also exhibit strong anisotropy. The anisotropy of the magnetic penetration depth in the uniaxial high-Tcmaterials is a direct consequence of the superconducting currents not being isotropic . In addition to the penetration depth exhibiting a large - anisotropy, recent measurements on untwinned single crystals show a small but significant - anisotropy .