- Decimus Junius Brutus Albinus (c.85-43 B.C.) and
- Marcus Junius Brutus (85-42 B.C.). Marcus Brutus was also called Quintus Servilius Caepio Brutus after his adoption.
Decimus Brutus was a remote cousin of Caesar. Ronald Syme* (20th century classicist and author of The Roman Revolution and an authoritative biography of Sallust) believes Decimus Brutus was the one who might have been Caesar's son. The mother of Decimus was Sempronia.
The mother of Marcus Brutus was Servilia, with whom Caesar had a long-term affair. Marcus Brutus divorced his wife Claudia in order to marry Caesar's fierce opponent Cato's daughter Porcia.
Marcus Brutus convinced Decimus Brutus to join the conspiracy. Then Decimus Brutus persuaded Caesar to go to the Senate despite Caesar's wife Calpurnia's warnings. Decimus Brutus is supposed to have been the third to stab Caesar. Afterwards, he was the first assassin to be killed.
It is reported that when Caesar saw Marcus Brutus approach to stab him, he pulled his toga over his head. Other reports include a memorable last line, possibly in Greek or the one that Shakespeare uses, "Et tu, Brute...." This is the Brutus attributed with the original of John Wilkes Booth's famous Sic semper tyrannis 'So always to tyrants'. Brutus may not have said it. Clearly, Marcus Brutus is the Brutus referred to as the most famous of Caesar's assassins.
Usually given as an objection to Caesar's being the father of Marcus Brutus -- although it would be just as valid or irrelevant with Decimus -- Caesar would have had to sire his son at around the age of 14. See: Was Caesar the Father of Brutus?
*"No Son for Caesar?" by Ronald Syme. Historia: Zeitschrift für Alte Geschichte, Vol. 29, No. 4 (4th Qtr., 1980), pp. 422-437