Abstract Baltic sediments have been studied by Behrens, Munthe, Küppers, Spethmann, Apstein, Sjostedt, Pratje and the writer. The following types of sediments have been observed: varved and non-varved late-glacial clays, gray and black, post-glacial muds, and sands. The organic content of late-glacial clays ordinarily is less than 1.3 per cent, and of post-glacial muds more than 3 per cent. Sediments containing intermediate quantities are scarce. This can be explained as a result of the changed balance between organic and inorganic sedimentation when the glacial period ended; the abundance of fresh detritus then suddenly ceased and inorganic sedimentation became very much slower than before; consequently, the relative amount of organic detritus increased. As most of the material was not subjected to biological analysis, it has not been possible to distinguish different ages among post-glacial sediments. Extremely fine-grained sediments occur in the Baltic. Non-varved late-glacial clays have medians from 0.40 to 0.62 micron, and post-glacial muds collected in the Baltic proper have medians from 0.58 to 1.6 microns. Non-varved late-glacial clays have a uniform composition over a wide area. The percentage of colloidal clay in total clay is about 5 per cent higher in post-glacial muds, showing that these sediments have been more exposed to weathering. This is attributed to the slower rate of settling and also the chemical and biological processes taking place within them as a result of the high content of organic matter. It is shown that Baltic water is under-saturated with respect to calcium carbonate. Yet carbonates do occur in the bottom deposits. In both late-glacial and post-glacial sediments, the carbonate content is ascribed to limestone detritus from deposits on the shores of and on the islands of the Baltic. The highest carbonate content, more than 40 per cent, was found in the southern part of the Gulf of Bothnia, on the bottom of which Silurian limestone is known to occur