While Iran has long been castigated for supporting Islamic terrorism around the world, they also have a terrorism problem at home. This is a result of Iran still being an empire. Only about half of Iranians are ethnic Iranians (or Persians, as many Iranians still call themselves). The rest of the Iranians are ethnic and religious minorities. It's the ethnic minorities that cause the most terrorism problems. The largest minority, Azeris (Turks) are a quarter of the population. Kurds are about seven percent, one percent are Baluchis, and three percent are Arabs. Another 14 percent are various other Turkic groups, or Indo-European groups related to the Persians.
Right across the border in the northwest is the nation of Azerbaijan, which used to be part of the Soviet Union. Independent for the last fifteen years, the existence of an independent Azerbaijan, run by Azeris, has inspired some separatist sentiments among Iranian Azeris. That's unfortunate, because Azeris have done well in Iran, and for the nearly two centuries that Russians ran Azerbaijan, it was the Iranian Azeris that believed they had the better deal. Many Iranian Azeris hold prominent positions in government, academia, business and the clergy. But ethnic Iranians still look down on the Azeris, and this often comes out into the open. Part of this is resentment, as for the last five centuries, Azeris have been overrepresented in the ruling class. Because of that, most Azeris live in and around the capital. But some of the Azeris living up bear the border with Azerbaijan, are acting up. This, literally, terrifies ethnic Iranians, for the ultimate nightmare is widespread unrest by the nations largest, and most able, minority. Iran also shares a border with Turkey, which considers itself the protector and "elder brother" of all the Turkic people. Ethnic Iranians blame unrest by Azeris on "Turkish agents," but there is no proof of that. However, many educated and affluent Azeris are unhappy with the religious dictatorship that runs Iran (even though many of the senior clerics running things are Azeris.)
The Kurds, like their kinsmen in Turkey and Iraq, have been rebelling for centuries. Currently, there are several organized separatist Kurdish groups in Iran. There have been lots of gunfire and explosions in the Kurdish areas of Iran (near the border with northern Iraq, where semi-independent Iraqi Kurds live.)
The Iranian Arabs are native to the areas of southwest Iran where most of the oil, but have not benefited as much as they believe they should have from all that wealth. Ethnic Iranians have a low opinion of Arabs, and rarely try to hide it. Separatist Iranian Arabs have been getting more violent since Iraq became independent. The Iranian Arabs are Shia, and now they see Iraqi Shia Arabs running Iraq. You know the rest.
The Baluchis, like the Pushtuns, Kurds and Tajiks, are Indo-European peoples like the Persians. The major difference is that the Persians (actually a coalition of "tribes" that have long since merged into one ethnic group) got themselves very organized and founded an empire thousands of years ago. The Persians have founded several empires, and regard their tribal kinsmen as a bunch of unruly cousins. The Baluchis have never fully accepted Persian domination. Most of the Baluchi people live next door in Pakistan, where a very violent Baluchi separatist movement is fighting the Pakistani government. This has encouraged the Iranian Baluchis to get restive as well.
The remaining Iranian minorities are quiet, for the moment. But there's little comfort in that, with terrorism on the rise among the subject peoples of the Iranian empire.