Caligula is assassinated on January 24, 41, at the Palatine Games in a broad-based conspiracy involving the Praetorian commander Cassius Chaerea and several Senators.
According to Cassius Dio, Claudius had beome very sickly and thin by the end of Caligula's reign, most likely due to stress.
A possible surviving portrait of Claudius from this period may support this.
There is no evidence that Claudius had a direct hand in the assassination, although it has been argued that he knew about the plot— particularly since he left the scene of the crime shortly before his nephew was murdered.
However, after the deaths of Caligula's wife, Caesonia, and daughter, it becomes apparent that Cassius intends to go beyond the terms of the conspiracy and wipe out the Imperial family.
In the chaos following the murder, Claudius witnesses the German guard cut down several uninvolved noblemen, including many of his friends.
He flees to the palace to hide.
According to tradition, a Praetorian named Gratus found him hiding behind a curtain and suddenly declared him princeps.
A section of the guard may have planned in advance to seek out Claudius, perhaps with his approval.
They reassure him that they are not one of the battalions looking for revenge.
He is spirited away to the Praetorian camp and put under their protection.
The Senate quickly meets and begins debating a change of government, but this eventually devolves into an argument over which of them would be the new Princeps.
When they hear of the Praetorians' claim, they demand that Claudius be delivered to them for approval, but he refuses, sensing the danger that would come with complying.
Some historians, particularly Josephus, claim that Claudius was directed in his actions by the Judean King Herod Agrippa.
However, an earlier version of events by the same ancient author downplays Agrippa's role — so it is not known how large a hand he had in things.
Eventually the Senate is forced to give in and, in return, Claudius pardons nearly all the assassins.
Claudius takes several steps to legitimize his rule against potential usurpers, most of them emphasizing his place within the Julio-Claudian family.
He adopts the name "Caesar" as a cognomen — the name still carries great weight with the populace.
In order to do so, he drops the cognomen "Nero" which he had adopted as paterfamilias of the Claudii Nerones when his brother Germanicus was adopted out.
While he had never been adopted by Augustus or his successors, he is the grandson of Octavia, and so feels he has the right.
He also adopts the name "Augustus" as the two previous emperors had done at their accessions.
He keeps the honorific "Germanicus" in order to display the connection with his heroic brother.
He deifies his paternal grandmother Livia in order to highlight her position as wife of the divine Augustus.
Claudius frequently uses the term "filius Drusi" (son of Drusus) in his titles, in order to remind the people of his legendary father and lay claim to his reputation.
Several coup attempts will be made during Claudius' reign, resulting in the deaths of many senators.
Shortly after his accession, when Appius Junius Silanus is governor of Hispania Tarraconensis, he is recalled to Rome and married to Domitia Lepida, mother of the empress Messalina.
He is treated with the greatest of distinction, but having refused the advances of Messalina herself, he is soon put to death by the emperor.
Messalina and Narcissus, one of the freedmen who forms the core of the imperial court, accuses him of plotting to assassinate Claudius, and claim that they had seen Silanus attempting to murder the emperor in their dreams.