The anisotropic character of interplanetary magnetic-field turbulence has been studied through the analysis of Cluster data. The full tensor of the mixed second-order structure functions has been used to quatitatively measure the degree of anisotropy and its effect on small-scale turbulence. Three different regions of the near-Earth space have been studied, namely the solar wind, the Earth's foreshock and magnetosheath. While in the undisturbed solar wind the observed strong anisotropy is mainly due to the large-scale magnetic field, near the magnetosphere other sources of anisotropy influence the magnetic-field properties.