The lecture course on radiation theory discusses the theory of blackbody radiation in thermal equilibrium leading to Kirchhoff’s law and to Planck’s radiation law. The course, held in summer 1912, is one in a series of related physics courses. For the summer term 1912 and for the winter term 1912/13, Hilbert announced courses on the “mathematical foundations of physics” in the university catalog. The *Ausarbeitungen* of these two lectures are entitled more specifically “Radiation Theory” and “Molecular Theory”. The title of “mathematical foundations of physics” served as an umbrella term for Hilbert’s interest both in foundational work and physics research. It indicated the expansion of his activities towards physics at this time.