Iran and the United States will hold talks Saturday in Rome, their second round of negotiations over Tehran’s rapidly advancing nuclear program.

A second round of nuclear talks between the United States and Iran is underway, as what both sides are looking for in a deal begins to take shape. Delegations from both countries met in Oman last weekend for talks mediated by the Gulf Arab nation.

Here's what to know about the letter, Iran's nuclear program and the tensions that have hurt relations between the U.S. and Iran since 1979.