Events will be held accross the capital today to commemorative 70 years after the liberation of Auschwitz-Birkenau in Poand.
Senior politicians and other dignitaries will join survivors for an event at Westminster with another event taking place the Imperial War Museum.
When Thomas Harding went to his great-uncle's funeral he learnt a lot more about him than he was expecting. He knew he came to London as a Jewish refugee, but he didn't know he'd been sent back to Germany to track down one of the world's worst-ever war criminals.
Thomas has now written a book about his great-uncle, an unknown hero of Auschwitz.Ruth Banks went to hear his incredible story.