Kampaku is a post held by a person who governs in the Emperor's stead.
Unlike with the Sessho, who exercised full authority as an agent of an infant or invalid emperor, in the case of the Kampaku the emperor held ultimate decision making authority. For the most part, a Sessho would go on to become a Kampaku. Also, because the Sessho Kampaku is customarily an agent of the Emperor, he typically did not participate in meetings (or in a decision) with the Daijokan (council of state of the Japanese imperial government) except in unusual circumstances such as when accompanying the Emperor; when the Daijo Daijin (Chancellor of council of state of the Japanese imperial government) or Sadaijin (Minister of the Left) was also serving as Sessho/Kampaku, the next minister in line would carry out government affairs as the Daijokan and chairman. In fact, the Kampaku had the right to inspect letters to the Emperor (more on this later); thus when there was political correspondence between the Emperor and Daijokan, the Kampaku was able to grasp and participate in its contents beforehand, giving him authority to control both the Emperor and the Daijokan without infringing upon the authority of the Emperor's edicts or the replies to them. This is called Sekkan politics (politics run by sessho and kampaku).
It comes from the words to entrust (kan) and to say (haku) the Emperor's words. The word Kampaku has its origins in the tradition followed in early Han China where all reports to Emperor Xuan were entrusted to the powerful statesman Huo Guang. This was so that Emperor Xuan, who feared Huo Guang's influence, could avoid being deposed by Huo Guang using negligence in government affairs as an excuse (Hakuriku is another name for Kampaku, because Huo Guang was 'Hakurikuko'). A previous Kampaku, who has passed on the position to a successor, was called Taiko by the Tang.
However, ironically in 887, in the imperial edict that Emperor Uta ordered TACHIBANA no Hiromi to write in a Kampaku appointment to FUJIWARA no Mototsune, Mototsune disagreed with the emperor regarding the meaning of the word 'Ako' and refused to participate in government for a while (the Ako Incident).
The first Kampaku was FUJIWARA no Mototsune, in the time of Emperor Uta (880). Qualification for the appointment was limited to line of regents in the lineage of the eldest son of the Northern FUJIWARA family, who were descendants of FUJIWARA no Michinaga. The source of that power was in the authority to view documents sent to the Emperor from the Daijokan and others before the Emperor did, called nairan. Consequently, there were cases wherein someone was not appointed Kampaku but gained the right of nairan to act as Kampaku and grab power - FUJIWARA no Michinaga was typical of this. On a related topic, the diary that Michinaga wrote is now called 'Mido Kampakuki,' but in fact Michinaga took the post of Sessho and never became Kampaku. It has been said that Michinaga did not become Kampaku because he saw the potential for the Kampaku to become politically powerless in a situation where the Emperor and Kampaku were opposed, because the Kampaku did not have political decision-making authority or the power to directly command the Daijokan or subordinate ministers. At the time the discord between Michinaga and Emperor Sanjo while he had nairan became a political issue.
Originally, a Sessho Kampaku running the government in place of the imperial regime was limited to assistance from a maternal relative because the Emperor was an infant. That is why the Sekkan household was established by sending a daughter to be the Emperor's queen, and having her bear a prince. Because of that, when Emperor Gosan-jo, who did not have any of the Sekkan house's blood, came to power, the influence of the Sekkan household began to decline. In particular the Cloistered Emperor Shirakawa brought about the demise of the age of Sekkan-centered politics with the start of his cloistered government after giving up the throne. From the Kamakura period on, because political power moved from the court to the warriors, the influence of the Kampaku on politics became ever weaker. However, Kampaku was always the highest rank for nobles, had great influence over within the Imperial court, and since it turned out a great many queens the Kampaku commanded authority and respect in every era.
After the Kamakura period, the main branch of the Northern FUJIWARA family split into the Konoe and Kujo families at the top, with Ichijo, Nijo and Takatsukasa families to make five regent families, and in successive generations the highest ranking among them was customarily appointed Sessho/Kampaku, continuing until the Meiji Restoration.
As an exception, in the Tensho period Hideyoshi TOYOTOMI took advantage of the Kampaku Dispute to be adopted by Sakihisa KONOE and succeed to Kampaku, becoming the first samurai Kampaku in Japanese history. When Hideyoshi acquired the Toyotomi clan, he became the first Kampaku that was not from the Fujiwara clan or one of the five regent families. Later, in order to realize a samurai Kampaku regime (samurai Kampaku system) from the Hashiba clan, Hideyoshi adopted his nephew Hidetsugu TOYOTOMI and made him Kampaku. However, while Hidetsugu was serving as Kampaku, government and the household were in the grip of the Taiko, Hideyoshi, and he later had a dispute with Hideyoshi and fell from grace. The Toyotomi regime continued after that, but Hideyoshi decided not to appoint a Kampaku until his son Hideyori TOYOTOMI reached adulthood. However, after Hideyoshi's death from the Battle of Sekigahara on, real power was transferred to the Tokugawa family, and the job of Kampaku was again given to the five regent families. Later, the Toyotomi family was wiped out in the Siege of Osaka and never recovered the post of Kampaku.
The Edo period Kampaku, under the Laws for Imperial Court and Nobles, as a rule had to be recommended by the Shogunate, and it would not be an exaggeration to say that the Kampaku, who was supposed to be the Emperor's first minister and the highest ranking noble, was in fact under control of the Shogunate. However, at that time, court meetings came to be held under the supervision of the Kampaku, the procedure was established for him to report, through the Imperial liaison, on the results of meetings that he had chaired on such important matters as the changing of eras and appointments to the Shogunate, and the Kampaku consequently came to have a lot of power within the court. Furthermore, the Kampaku alone among nobles was required to visit the Imperial palace daily (to put it another way, the power of those not required to do so seemed less in comparison the Kampaku's), and throughout the Edo period appointment to Daijo Daijin was limited to those who had been Tokugawa clan, Seii taishogun (literally, "great general who subdues the barbarians") and Sessho/Kampaku, so the quality of the reception at the Imperial palace was on another level. Moreover, there were many Kampaku who were able to send their daughter to be the Shogun's official wife (Midaidokoro), and there were some who had a certain influence in the Shogunate (Motohiro KONOE, for example). From the Meiji period on, the positions of Sessho, Kampaku and Seii taishogun were abolished, and the history of the Kampaku came to an end together with the Shogunate. After that, only the Sessho has been revived, designated under Imperial Household Law for only princes and other members of the Imperial family to act as an agent in carrying out the Emperor's official duties, and continues to this day.