Who replaced Caesar?

Julius Caesar was assasinated 44 BC by a group of senators, led by Marcus Junius Brutus and Gaius Cassius. Caesar's follower was his adopted heir, Octavian (or Octavius) (63 B.C.- A.D. 14), perhaps even better known for the name Augustus.

Octavian was aged only 19 at the time of Caesar's death and proved to have considerable political skills.

After Caesar's assassination, a political alliance, known as the Second Triumvirate, was formed in 43 BC by Octavian, Mark Antony and Lepidus.

The triumvirate was eventually torn apart: disagreement between Octavian and Antony erupted into civil war in 31 BC. Antony was defeated by Octavian at the naval Battle of Actium. He committed suicide, and his lover, Cleopatra, killed herself shortly thereafter.

Octavian became the first Roman emperor under the name Caesar Augustus.

Sources:
Pekka Tuomisto: Rooman diktaattorit
Wikipedia