The Agiad dynasty was one of the two royal families of Sparta, a powerful city-state of Ancient Greece. The Agiads were seniors to the other royal house, the Eurypontids, with whom they had an enduring rivalry. Their hypothetical founder was Agis I, possibly the first king of Sparta at the end of the 10th century BC, who gave his name to the dynasty. The last Agiad king was Agesipolis III, deposed by the Eurypontid Lycurgus in 215 BC. Their most famous member was  Leonidas I, known for his heroic death at the Battle of Thermopylae in 480 BC. 
 In order to explain the peculiarity of the Spartan two kings, the Spartans elaborated a legend saying that Aristodemos—the first king of Sparta—had twins, Eurysthenes and Prokles. Since the Spartans did not know who was born first, they opted for a diarchy, a college of two kings with the same power; Eurysthenes being the first Agiad, Prokles the first Eurypontid.
 Modern scholars consider instead Agis I and Eurypon to be the founders of each dynasty, as they give their name to their descendants, not the mythical twins. The two dynasties were however not related until the Hellenistic era and the Eurypontids reached royal status much later than the Agiads. As a result, in order to balance the two royal lines, several names were inserted in the list of Eurypontid kings, such as Soos (meaning "stability"), Prytanis and Eunomos (said to have ruled at the same time as Lycurgus). Thus, while the Agiads might have ruled from the end of the 10th century, the Eurypontids only received the kingship in the beginning of the 8th century at the earliest.
 It is probable that the two dynasties came to rule jointly under the kings Archelaos (Agiad) and Charillos (Eurypontid) in the 8th century, as a result of the synoecism that created the polis of Sparta. The city was composed of five villages (Pitana, Mesoa, Limnai, Kynosoura, Amyklai), the latter of which merged with the other four after the initial synoecism. The Agiads had their burial ground located in Pitana, while the Eurypontids were in Limnai, which suggest that the dual monarchy was created when the four villages merged. Archelaus and Charilaus are the first kings of Sparta that are considered together in ancient sources: following the oracle of Delphi, they destroyed and conquered Aigys, in the northwest of Sparta. The connection of the Spartan kings with Herakles likely dates of the same period, which also witnessed the construction of the Menelaion, a heroon to Menelaus.
 The genealogies given by the Greek writers Herodotus and Pausanias remain highly suspect before the 5th century, as it is not conceivably believable to have 16 direct successions (from father to son) from Eurystenes and Prokles. A lot of successions must have been collateral, especially when considering that of the 26 successions that took place after 491, only 14 were from father to son. Moreover, ancient chronologies produce an average length of 40 years per reign, which is far too long and a consequence of the descent from Herakles myth. Paul Cartledge suggest an average length of 30 years per generation, thus giving a regnal date of c. 930–900 for Agis I, founder of the Agiads. These dates relate well with the archaeological evidence.
 Spartan kings are shown in bold, all dates BC.
 c.815–c.785