Cassius Dio
The bulk of what is known of Caligula comes from Suetonius and Cassius Dio. Suetonius wrote his history on Caligula 80 years after his death, while Cassius Dio wrote his history over 180 years after Caligula's death. Cassius Dio's work is invaluable because it alone gives a loose chronology of Caligula's reign.