Among the most important questions in fundamental physics and cosmology are the origin and the masses of fundamental particles, in particular the neutrino masses. KATRIN will allow a model-independent measurement of the neutrino mass scale with an expected sensitivity of 0.2 eV/c2 (90% CL). KATRIN will use a source of ultrapure molecular tritium and is currently being commissioned at KIT, thereby making use of the unique expertise of the Tritium Laboratory Karlsruhe. This paper presents the status of the KATRIN experiment, with the focus on the activity monitoring of the KATRIN tritium source.