兩漢皇帝没有子嗣的情况頻繁發生，皇位繼承人多從宗親藩王中擇選。外藩入繼不僅關係到禮法的依據和施行，還牽動著當朝的政治神經。由於漢初禮法多未定型，有關外藩入繼以及入繼者如何尊封本生父母，没有明確固定的禮法規範。在入繼皇帝的尊親問題上，論者各執一詞，引發數次禮制論爭，甚至造成政治動盪，西漢政權的覆亡亦與此有所關聯；外藩入繼的禮法規範在光武帝劉秀的處理中才形成範式，在東漢後期逐漸定型。本文將就兩漢外藩入繼的情况加以論述，並聯繫政治角力和禮法爭議兩方面，就兩漢歷次外藩入繼事件的前因後果和複雜情况加以分析。

When Han-dyansty emperors deceased without any male offspring, a regnant-monarch was chosen as the heir to the throne. Adopting a regnant-monarch into the royalty bore association with the implementation of rituals and affected court politics. However, there were no institutionalized rituals for adopting a vassal king or conferring honorific titles to his biological parents in the early Han dynasty, which resulted in many debates over the ritual appropriateness. The confusion had led to political turmoil insofar as it contributed to the fall of the Western Han regime to a certain extent. Liu Xiu (Emperor Guangwu of the Eastern Han dynasty) finally institutionalized and confirmed such rituals. This paper discusses the adoption of a regnant-monarch into royalty in the Han Dynasty. It analyzes the causes and complexities in various such cases in history from the perspectives of political struggles and ritual disputes.