Twenty years after the discovery of neutrino oscillations established non-zero neutrino mass, the absolute neutrino-mass scale remains unknown. The KArlsruhe TRItium Neutrino experiment (KATRIN) is designed to improve the current direct limit on this mass scale by an order of magnitude, with a projected sensitivity of 0.2 eV/c2 at the 90% confidence level. To achieve this, KATRIN will perform high-precision spectroscopy of the endpoint region of the tritium beta-decay spectrum, using a high-intensity, windowless gaseous tritium source and a high-resolution electrostatic spectrometer. In this talk, I will review the theoretical basis for a tritium-based neutrino-mass measurement; explore some of the experimental challenges addressed by the KATRIN collaboration; and share early results from the commissioning of the experiment, including KATRIN's first tritium runs.