650 California Street, also known as the Hartford Building, is a 34-story, 142 m (466 feet) office tower on the northwestern edge of San Francisco's Financial District.

[4] It is architecturally significant, featuring a tall modernist lobby, high ceilings, and an exterior skeleton of floor-to-ceiling windows recessed into a square gridwork of precast white reinforced concrete.

[4] It is one of 39 San Francisco high rises reported by the U.S. Geological Survey as potentially vulnerable to a large earthquake, due to a structural deficiency.

[10] 650 California Street passed the certification requirements for the Gold Level of the Leadership in Energy and Environmental Design (LEED) rating in June 2011.

[citation needed] After Elon Musk's acquisition of Twitter, the company ceased paying rent for its 30th floor office rental.