Dachau, the first camp.
Auschwitz, where more than 1 million Jews, Roma (Gypsies), Poles, Soviet prisoners of war and others were killed.
But as horrific as they were, they were only three camp complexes in a system of more than 850 ghettos, concentration camps, forced-labor camps and extermination camps that the Nazis established during the 12 years Adolf Hitler was in power.
By the end of World War II in 1945, the death camp system stretched from France and the Netherlands in the west to Estonia, Lithuania and Poland in the east.
There were 20 main concentration camps, many of which had many subcamps, according to Geoffrey Megargee, the editor of the U.S. Holocaust Memorial Museum's Encyclopedia of Camps and Ghettos.
Many of them combined the most dehumanizing and degrading characteristics of prison and slave labor camps. In some, Nazi doctors carried out depraved experiments, while others were primarily transit camps -- places to hold Jews and other "undesirables" until they were sent to other camps.
There were also four main extermination camps -- Belzec, Chelmno, Sobibor and Treblinka -- devoted solely to killing everyone who passed through their gates. Treblinka nearly rivaled Auschwitz in the sheer number of people who were murdered there.
Between 870,000 and 925,000 people were killed at Treblinka in Poland, 170,000 died at Sobibor, at least 152,000 were murdered at Chelmno, and about 434,500 Jews were killed at Belzec.
In all, about 6 million Jews and millions of others died in the Holocaust.
But the exact numbers of dead will probably never be known, nor will the total number of people held prisoner in Nazi camps.
The Nazis were famously good record keepers, but they managed to destroy the records of some camps before the Allies liberated them.
The infamous Lublin-Majdanek camp, which -- like Auschwitz -- had elements of both concentration camp and extermination camp, was one where records were destroyed.
About 80,000 to 110,000 people died in the main camp, but it's impossible to know how many prisoners it held in its time.
Bergen-Belsen was another camp whose overlords erased as much history as they could.
Historians may never determine how many prisoners the camp held in its time, but it's clear that about 50,000 people perished there, including Anne Frank, who died of disease mere weeks before the camp was liberated.