This Commission was organized at the Stockholm meeting of the Union in 1938, and now after ten years has the opportunity to make its first report. The rearrangement of the fields of the different Commissions at Copenhagen in 1946 gives Commission 34 some responsibility for the entire volume of space between the stars, if not between the galaxies. Since the presence of interstellar material in any region of space may be revealed by emission, reflection, scattering, or absorption of radiation, the study of this material falls under or overlaps the activities of half a dozen other Commissions of the Union. Because of this extent of the relations of the Commission we may consider again the directions in which international co-operation may function to advantage. As pointed out by Schlesinger when the Union was organized, such co-operation is valuable in matters of convention; in avoidance of duplication; and especially in investigations too large for one institution or even one country to undertake.