Early life and rise to power (161–180)
Commodus was born on 31 August AD 161 in Lanuvium, near Rome. He was the son of the reigning emperor, Marcus Aurelius, and Aurelius's first cousin, Faustina the Younger, the youngest daughter of Emperor Antoninus Pius, who had died only a few months before. Commodus had an elder twin brother, Titus Aurelius Fulvus Antoninus, who died in 165. On 12 October 166, Commodus was made Caesar together with his younger brother, Marcus Annius Verus. The latter died in 169 having failed to recover from an operation, which left Commodus as Marcus Aurelius's sole surviving son.
He was looked after by his father's physician, Galen, who treated many of Commodus' common illnesses. Commodus received extensive tutoring by a multitude of teachers with a focus on intellectual education. Among his teachers, Onesicrates, Antistius Capella, Titus Aius Sanctus, and Pitholaus are mentioned.
Commodus is known to have been at Carnuntum, the headquarters of Marcus Aurelius during the Marcomannic Wars, in 172. It was presumably there that, on 15 October 172, he was given the victory title Germanicus, in the presence of the army. The title suggests that Commodus was present at his father's victory over the Marcomanni. On 20 January 175, Commodus entered the College of Pontiffs, the starting point of a career in public life.
In April 175, Avidius Cassius, Governor of Syria, declared himself Emperor following rumours that Marcus Aurelius had died. Having been accepted as Emperor by Syria, Judea and Egypt, Cassius carried on his rebellion even after it had become obvious that Marcus was still alive. During the preparations for the campaign against Cassius, Commodus assumed his toga virilis on the Danubian front on 7 July 175, thus formally entering adulthood. Cassius, however, was killed by one of his centurions before the campaign against him could begin.
Commodus subsequently accompanied his father on a lengthy trip to the Eastern provinces, during which he visited Antioch. The Emperor and his son then traveled to Athens, where they were initiated into the Eleusinian mysteries. They then returned to Rome in the autumn of 176.
Joint rule with father (177)
Marcus Aurelius was the first emperor since Vespasian to have a legitimate biological son and, though he himself was the fifth in the line of the so-called Five Good Emperors, each of whom had adopted his successor, it seems to have been his firm intention that Commodus should be his heir. On 27 November 176, Marcus Aurelius granted Commodus the rank of Imperator and, in the middle of 177, the title Augustus, giving his son the same status as his own and formally sharing power.
On 23 December of the same year, the two Augusti celebrated a joint triumph, and Commodus was given tribunician power. On 1 January 177, Commodus became consul for the first time, which made him, aged 15, the youngest consul in Roman history up to that time. He subsequently married Bruttia Crispina before accompanying his father to the Danubian front once more in 178. Marcus Aurelius died there on 17 March 180, leaving the 18-year-old Commodus sole emperor.