The belief in their existence is based on historical speculations dating back to the 2nd and 1st century BCE. Confucius, who lived earlier than that, only mention Yao and Shun. The number five reflects the Chinese system of cosmological correspondences, and since the number five is also related to the five elements, one element is allocated to each of the five emperors.
The five legendary emperors, successors of the San-huang. They are said to have ruled China between 2697 and 2205 BCE or -- according to an alternative calendrical calculation -- 2674 and 2184 BCE. They are Huang-di (the Yellow Emperor), Juan Xu, Gu, Yao, and Shun.
The name of Wu-di in traditional Chinese format.