It's vexed San Francisco's leaders for more than three decades.

The last seven mayors have all tried to tackle the city's persistent homeless problem, introducing scores of different plans and task forces, each of which ultimately failed to markedly reduce the sheer number of people living on the streets. Consistently ranked among the most pressing concerns among the city's residents, the issue has proven a political lightning rod in a city known for its liberal views, innovative spirit and wide income gap.

There have, of course, been some successes along the way, though progress is often hard to notice. Over the last decade, the city has cumulatively moved about 20,000 people off its streets. But that's come at a cost of about a quarter billion dollars a year, and the day-to-day population hasn't budged -- it's actually risen slightly, to almost 6,700, according to a 2015 count -- even as the city now spends nearly a quarter-billion dollars a year on homeless services. The following timeline describes the different ways each of the the city's last six mayors tried to tackle the issue. Photos used with permission from the San Francisco Chronicle.

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TIMELINE: The Frustrating Political History of Homelessness in San Francisco

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