The Ludwig Forum is Aachen's main contemporary art museum, housed in a former umbrella factory east of the old town. Its collection grew from the Ludwig holdings and gives the city a sharper modern counterpoint to the cathedral, treasury, and imperial history that dominate most visits.
Come for postwar and contemporary art in generous industrial spaces rather than a small old-master gallery. The museum is especially useful if you are spending more than a few hours in Aachen and want to see the city looking outward, not only backward to Charlemagne.