In this essay I examine how the periodic system or table was introduced in Denmark in the late nineteenth century, how it was used in chemical textbooks, and the way it was developed by a few of the country’s scientists. Danish chemists had in the period an international orientation, which helped them in getting acquainted with Mendeleev’s system and appreciating its strength. The main reason they felt the system to be attractive was its predictive force, especially its prediction of new elements and ability to accommodate new chemical knowledge. I pay particular attention to the work of Hans Peter Jørgen Julius Thomsen (1826–1909), which is an important example of “neo-Proutean” attempts to understand the periodic system in terms of internally structured atoms. Moreover, I direct attention to Mendeleev’s connection to Danish science by way of his membership in the Royal Danish Academy of Sciences and Letters. Thomsen’s speculations of composite atoms as the ultimate cause of the periodicity of the elements were vindicated by the new developments in atomic theory. A semi-quantitative explanation was offered by Niels Bohr (1885–1962) in 1913, and in subsequent refinements of his atomic model he came close to an explanation of the entire periodic system. The essay briefly considers Bohr’s work on the periodic system in its local context, including its relation to the earlier ideas of Thomsen. In order to appreciate how the periodic system of the elements was received in Denmark, it will be helpful to provide some basic information of the country’s chemical landscape. In the period here considered, approximately 1870–1920, Denmark was a small country, scientifically and culturally almost completely dominated by its capital, Copenhagen. As far as chemical research and education was concerned, the most important institutions were the University of Copenhagen, the Polytechnical College, the Royal Veterinary and Agricultural College, and the Pharmaceutical College, all located in Copenhagen. Although the number of chemists grew rapidly during this period, only a few of them were trained at the University and even fewer had an interest in the more theoretical aspects of the chemical sciences.