Auschwitz-Birkenau is synonymous with the Holocaust. More than a million Jews, and many Poles and Roma, were murdered here by German Nazis during WWII. Both sections of the camp – Auschwitz I and the much larger outlying Birkenau (Auschwitz II) – have been preserved and are open to visitors. It's essential to visit both to appreciate the extent and horror of the place. From April to October it’s compulsory to join a tour if you arrive between 10am and 3pm.