By placing Stolpersteine ("stumble stones") outside the victims' last known address, 75-year-old Gunter Demnig aims to draw attention to the fate of individuals in the Holocaust.
Nearly 100,000 cobblestones later, they can be found in 30 countries across Europe, from Finland to Italy, Hungary, Russia and Ukraine.
"I never dreamed of this," Demnig said, saying he had expected a few hundred or maybe 1,000 stones.
They commemorate all groups of the Nazis' victims, including Jews, Sinti and Roma, political opponents, gay people and "antisocial elements", or criminals.
Both were murdered," said Demnig as prepares to lay two stones outside the house in Cologne where they lived.