- After the death of Alexander, the empire was divided into three large kingdoms, the Seleucid, Parthian, and Sasanid Empire.
- The number one best realm went to Seleucus who was a commander of an elite corps of guards in Alexander's army.
- Seleucus and his successors kept the Achaemenid systems of administration and taxation, as well as the imperial roads and postal service. Seleucids founded new cities throughout their kingdom and attracted Greek settlers to occupy them.
- Satraps often revolted against Seleucid rule, and tried to establish their own independence.
- Roman conquerers ended the Seleucid Empire.
- Occupied the region of eastern Iran around Khurasan.
- They continued to keep many of the nomadic people's customs and traditions.
- Did not have a centralized government, and organized themselves politically through leaders who met in councils and determined the policy for all joined groups.
- Skillful warriors
- In order to effectively resist nomadic invasions, Parthians fed their horses alfalfa during the winter which made them grow larger and stronger than the nomadic people's horses. The larger horses would then be able to support the weight of heavily armed warriors, thus making the Parthian's horse a stronger force and act as a shield.
- Mithradates I (Parthian's greatest conquerer) created a mighty empire.
- The Parthians governed through satraps, employed Achaemenid techniques of administration and taxation, and built a capital city at Ctesiphon on the Euphrates River.
- Gave their clan leaders a great deal of authority and responsibility.
- Internal rebellion with the satraps led to the fall of the Parthian Empire.
- The Sasanids came after the Parthians in 224 CE.
- Carried on the tradition of imperial rule.
- The Sasanid king delivered strong rule from Parthia to Mesopotamia, rebuilt a complex system of administration, and rebuilt and improved numerous cities.
- Sasanid merchants traded often with the east and west and introduced Iran to crops like rice, sugarcane, citrus fruits, eggplant, and cotton that came over the trade routes from India and China.
- Stabilized their western frontier and created a series of buffer states between their empire and the Roman empire.
- Defeated several Roman armies and had the prisoners construct roads and dams using their famous Roman engineering skills.
- After Shapur I the died, the Sasanids did not expand militarily.
- Conflicts between the Kushan, Roman, and Byzantine empire to overcome each other seriously weakened the Ssasanid empire.
- The empire came to an end when Arab warriors killed the last Sasanid ruler, took over the territory, and added it to their Islamic empire.