The study of the distribution of matter in the Universe is at the forefront of modern astronomy, and stimulates numerous research programmes both in theory, observation and experiment. The distribution of galaxies has a cell-like structure resulting from the alternation of sheets and voids with the topological properties of a ‘sponge’. The large diameter of the voids (150 million light-years) and the high degree of isotropy of the microwave background radiation define the reference frame for the theoretical models for the formation of the large-scale structure. The essential ingredients of these models are: the clustering of matter under the gravitational attraction, the initial matter fluctuations which left their imprint into the cosmic microwave background radiation and a large amount of dark matter filling the voids and walls of galaxies. Measurement of the statistical properties of the large-scale structure, and determination of the scale at which it becomes homogeneous provide direct tests of the Big Bang model. Major advances are expected by the end of the century from the largest telescopes and their sophisticated instruments. © 1997 by John Wiley & Sons, Ltd.