The number of pages allocated to the commission report has been very limited and certainly not sufficient to cover in any exhaustive manner the wide range of topics relevant to cosmology and to provide also extensive bibliographies. Because of the vast amount of material to be covered, the report is based on a number of contributions from different colleagues who have been asked to highlight the main trends in the triennium (mid 1984 - mid 1987), together with a list of references sufficiently comprehensive to serve as a guideline for further reading. Unfortunately, two of the expected contributions did not reach me in time for inclusion in the report, and consequently topics such as the large scale structure and streaming motions, the clusters of galaxies and the counts of extragalactic radio sources are not included. However, it is my understanding that a large portion, if not all, of these topics will be covered in the reports of Commissions 28 and 40, and if true, this will at least avoid unnecessary overlaps. It should also be mentioned here that several proceedings of very recent IAU conferences provide excellent, updated and exhaustive reviews of the research work relevant to cosmology.