The distribution of galaxies on scales larger than around 30 million l.y. Galaxies are not scattered randomly in space but are grouped in clusters and superclusters of various sizes. They also sometimes lie in extended chains called filaments, or in flattened structures such as the Great Wall. Contrasting with these areas are vast regions where galaxies are scarce, known as voids, which appear to be roughly spherical. The largest structures known are around 300 million l.y. across. If the cosmological principle is correct, astronomers should eventually reach a scale at which the Universe becomes roughly homogeneous. Surveys of large-scale structure demonstrate that the Universe is close to homogeneous on scales larger than this.