Next to Jew, the only other group that was systematically targeted for extermination - I'll return to this later - were people whom the Nazis defined as "gypsies", most of them either Roma or - to a lesser degree - Sinti.

When talking about the the genocidal actions and intentions of the Nazi regime, the process of the targeted killing of the so-called gypsies is often referred to as Porajmos (the devouring, the fragmentatio or the destruction) vis a vis the Shoah as the targeted murder of Europe's Jews.

The reasoning behind targeting the so-called gypsies lay within a sort of conflation between a racism on an ethnic as well as a racism (for lack of a better term) on a social basis. The people who were lumped under this category were seen as hereditary "asocial", their völkisch heritage thereby threatening the German people by spreading criminal and socially deviant behaviour through their genes/blood so to speak. If scaled, they represented to the Nazis a lesser threat than the Jews who according to Nazi pseudo-scientific and paranoid racial theories were responsible for Bolshevism, Capitalism and all other sorts of dangers to the Germans, but these so-called gypsies represented a grave threat nonetheless.

Similar to anti-Semitism, Antiziganism has a longer tradition to look back to in Europe. In Germany and probably other states in Europe too, these long traditions were actualized when in the latter years of the 19th century criminology mixed with racial theory. The emergent German nation state as well as some of the preceding states followed a criminal discourse that placed criminality in a seemingly "natural" association with vagaboundage/having no fixed place of living. This merged with the at the time highly popular theory of criminal behaviour being hereditary as well as racially herditary and thus emerged the modern version of Antiziganism that targeted so-called gypsies as people who trhough their live style were associated with criminality all along.

With the Nazis' rise to power this gained a new quality. Additionally to the police already keeping files and very close tabs on the communities of Roma and Sinti in Germany, they were now defined as "artfremd" (foreign to the species or in this case the German racial community) along with the Jews as per the most prominent commentary of the Nuremberg Laws of 1935.

While the subsequent years already saw persecution of Roma and Sinti and other people defined as "gypsies" in such actions such as mass deportations to Concentration Camps in the course of the new policy of preventive crime fighting the Gestapo instituted in 1936/37, the systematic persecution of the so-called gypsies got underway paralell with that of the Jews. When the Nazis started concentrating Jews in Ghettos in 1939 and deporting German Jews to said Ghettos starting on a large scale in 1941, Roma and Sintin were always included in these actions.

Similarly, with the Aktion Reinhard Camps, where especially in Treblinka a to this day unknown number of Roma and Sinti were killed. Following Himmler's "Gypsy Decree" of December 16, 1942, all "gypsies" and mixed "gypsies" in Germany especially but also the other controlled territories were to be arrested and sent to the special "gypsy" camp in Auschwitz, that was part of the Auschwitz-Birkenau complex. This camp also became to be known as the so-called "Familienlager" (family camp) because in contradiction to the Jews deported to Birkenau, children and elderly were not killed on arrival and men and women not separated. However, there were at least three separate mass killing actions in the Zigeunerlager in Auschwitz and of the approx. 22.000 deported there, at least 19.000 died.

Despite these orders, most of the Roma and Siniti who perished through the Nazis' policy were shot in Eastern Europe. The Einsatzgruppen as well as the Wehrmacht reprisal shootings or the shootings done by the Police Battalions in Poland always included so-called gyspies alongside Jews. For this reason and the reason that this was ignored by historical scholarship for a very long time (old stereotypes run very deep), it is really hard to come to a number concerning all the Roma and Siniti murdered by the Nazis. For a long time, the number of 500.000 was seen as the best estimate we have but more recent scholarship suggests that it is more along the lines of something inbetween 200.000 and 500.000 (out of an estimated total of about 700.000-900.000 Roma and Sinti in Europe at that time).

Returning to the issue of systematic as I mentioned in the beginning, what sets the murder of Jews and Roma and Sinti apart was that wherever and whoever they were, the Nazis' agenda was to murder them because of the alleged threat they posed to the German racial community as a whole. Aktion Reinhard is a good indication for this: While test gassings were done with Soviet POWs, the only victims sent to these camps that were entirely designed to kill the vast majority of people sent there were Jews and Roma and Sinti.

As /u/Kugelfang52 has already so informatively and well demonstrated, homosexuals for example were not targeted for extermination. The only other group that was targeted for mass killing to a certain extent were mentally handicapped and disabled people in the course of the centralized T4 action, a program designed to rid Germany of the economic burden of the lives "unworthy of living". It consisted of mentally handicapped and disabled people being deported from the homes they were in to six centralized facilities in Germany to be killed. This lasted from 1939 to 1941 when the centralized program was discontinued and became decentralized, i.e. doctors were encouraged to murder their mentally handicapped or disabled patients on their own accord.

Now, the T4 program was a very important precursor for the Holocaust in that it was the first time the Nazi government really crossed the line of using concerted lethal force as a political means in regards to a minority group but the way it was executed in 39-41 it lacked the sytematicity of the later Holocaust against Jews and so-called gypsies. At that point in time not all mentally handicapped and disabled people were targeted but only those in homes, i.e. only those who posed an economic burden on the Reich. If the Nazi leadership had not been forced to stop that program because of the public protests against it or had the Nazis won the war, it is not unlikely they would have expanded this program to include all mentally handicapped and disabled people but as things stood, they did not.

As an interesting side-note, one group that in the 1920s and in the beginning of the Nazi rule was very much focused on and planned to be systematically persecuted were the Freemasons. Particularly in the SS, Freemasons were seen as in league with the Jews in their perceived conspiratorial attack on Germany. After the dissolution of most lodges however, this agenda just sort of disappeared never really to be brought up again.

Sources:

• Till Bastian: Sinti und Roma im Dritten Reich. Geschichte einer Verfolgung, C. H. Beck, München 2001.

• Wlaclaw Dlugoborski (Hrsg.): Sinti und Roma im KL Auschwitz-Birkenau 1943–1944. Vor dem Hintergrund ihrer Verfolgung unter der Naziherrschaft. Oświęcim 1998.

• Crowe, David; Kolsti, John (eds.). The Gypsies of Eastern Europe. Armonk.

• Heuss, Herbert; Sparing, Frank; Fings, Karola; et al., eds. (1997). The Gypsies during the Second World War. 1 From "Race Science" to the Camps.

• Milton, Sybil (1992). Nazi Policies Toward Roma and Sinti, 1933-1945.

• Lewy, Guenter (2000). The Nazi Persecution of the Gypsies. Oxford University Press.

• Browning, Christopher (2005). The Origins of the Final Solution: The Evolution of Nazi Jewish Policy, September 1939 – March 1942.

• Friedlander, Henry (1995). The Origins of Nazi Genocide. From Euthanasia to the Final Solution. University of North Carolina Press, Chapel Hill & London.

• Klee, Ernst (1983). Euthanasie im NS-Staat. Die Vernichtung lebensunwerten Lebens (in German). Frankfurt am Main: Fischer Taschenbuch Verlag.

Yes. Although many groups were targeted during the holocaust, homosexuals and the Roma people come to mind. Nazi Germany was particularly harsh on those the regime did not believe could adapt to the new system, and the Roma were seen as completely racially 'impure.' Yad Vashem notes that in Poland alone, two thirds of the Roma population were exterminated, including in Concentration camps like Dachau.

An essential part of living under the Nazi regime was the ability to bear and conceive healthy, racially pure, 'Aryan' babies. It was not believed that Gay men had this ability, so their persecution was more or less inevitable, especially as the Nazi party was strictly conservative and homophobic, despite initially working with allies like Ernst Rohm, who was more or less openly gay.

Sources:

The Shoa Resource Centre,, The International School for Holocaust Studies: 'Gypsies'