Japan is home to the oldest continuous hereditary monarchy in the world, the Imperial House of Japan. Japanese tradition traces the dynasty back to Emperor Jimmu, said to have founded the imperial line in 660 BCE, while Emperor Naruhito is counted as its 126th monarch.
The earliest part of that traditional chronology is not historically verifiable. Scholars generally regard Jimmu and several early emperors as legendary, and secure historical evidence for the imperial line begins much later, around the sixth century CE. Even with that caveat, Japan's monarchy is widely described as continuous because the throne has remained within the same imperial dynasty.
The emperor's political role has changed dramatically. For centuries, shoguns and court nobles often exercised practical power while the emperor retained ritual and symbolic authority. After the Meiji Restoration in 1868, imperial power was expanded, but Japan's postwar Constitution defines the emperor as a symbol of the state and the unity of the people.
The succession is hereditary and traditionally male-line. This makes Japan distinct from modern constitutional monarchies whose royal houses are comparatively recent, even though the exact age and uninterrupted nature of Japan's earliest lineage remain subjects of historical discussion.