The present account refers mainly to research work published in the period 1973–1975. Due to the limited space available, and the rapidly increasing number of contributions in the field of Commission. 28, it has not been possible to write an all-inclusive report. As in the previous IAU volumes, the report is not intended as a repetition of the summaries given in the Astronomy and Astrophysics Abstracts. In stressing the importance of certain areas of research, the personal views of the writer(s) cannot be entirely avoided.

It has become a tradition for Commission 33 to invite several authors to contribute to the Report. The scope of the Commission is so wide that it is beyond the possibilities of any one individual to do this work without assistance. B. Westerlund contributed Section 2, S. W. McCuskey Sections 3 and 4, T. Elvius the paragraph on high-latitude studies, F. J. Kerr Sections 3, 4 and 5B, J. Einasto Sections 6B and D, and a summary of the current research by Soviet astronomers, K. C. Freeman Sections 6C, E. L. Woltjer Section 6F, and L. Perek Sections 1, 5A, and 6A. Section 6B contains this time a paragraph on the Galactic Corona as seen from the point of view of galactic models, while Section 4 contains a paragraph on the structure of the Galactic Halo.

The subject of interstellar matter is one of the most active fields of astronomical research to-day. Observational information spans the full electromagnetic spectrum from long radio waves to gamma rays and includes cosmic-ray particles. Results of chemical research find as much application now as mathematical methods. Interstellar matter plays a leading role in galactic and extragalactic research, and contributes increasingly to stellar astronomy and in solar system studies.

This triennium has been quiet but industrious. Information and understanding about star clusters and associations have advanced in a quantitative way, on a broad front that encompasses all their aspects. The tabulations given below refer to many of the well-known objects, for which improved data are now available, plus an impressive number of objects that heretofore have been little more than catalogue entries. Clusters and associations have always been the source, the stage, and the touchstone for the investigation of star birth, stellar evolution, populations, and galactic structure, and the data of the 1976 report of Commission 37 will figure in the scientific advances reported in 1979 by several other IAU Commissions.

A major procedural advance in the determination of the H0 and Ω has been that the problem is now being attacked from many different points of view and to some extent the observations are converging on preferred values of H0 and Ω (Ω = density parameter = 8πGρ0/3H0 where ρ0 is the mean density of matter in the Universe and H0 is the Hubble constant; Ω = 2q0 where q0 is the deceleration parameter). The classical approaches through the redshift-magnitude relation for the most massive galaxies in clusters suggest a value of H0 = 60 km s-1 Mpc (see the review by Tammann in IAU Symp. 63).