But really, who is this guy?
First of all, he's the Mayor of San Antonio, Texas. And secondly, he has a twin brother - Joaquin Castro - who currently serves in the Texas Legislature and is running for U.S. Congress this year.
Joaquin on the left and Julian on the right
Both of them graduated from Stanford, went to Harvard Law School, and then returned to San Antonio to get involved in public service.
To give you an idea of what's in store at the Democratic Convention, here's the end of Mayor Castro's speech to the Texas Democratic Convention this year.