1. VIRGINIA TECH KILLINGS: Seung-Hui Cho, 23, who had avoided court-ordered mental health treatment despite a history of psychiatric problems, killed two fellow students in a dormitory on April 16, detoured to mail a hate-filled video of himself to NBC News, then shot dead 30 students and professors in a classroom building before killing himself. It was the worst mass shooting in modern U.S. history.

2. MORTGAGE CRISIS: A record-setting wave of mortgage foreclosures, coupled with a steep slump in the housing market, buffeted financial markets, caused multibillion-dollar losses at major banks and investment firms, and became an issue in the presidential campaign.

3. IRAQ WAR: The "surge" that sent more U.S. troops to Iraq was credited with helping reduce the overall level of violence. But thousands of Iraqis and hundreds of U.S. personnel were killed nonetheless during the year, and Iraqi political leaders struggled to make meaningful progress toward national reconciliation.

4. OIL PRICES: Oil prices soared to record highs, at one point reaching nearly $100 a barrel. The high prices, which burdened motorists and owners of oil-heated homes, nudged Congress to pass an energy bill that ordered an increase in motor vehicles' fuel efficiency.

5. CHINESE EXPORTS: An array of Chinese exports were recalled, ranging from toys with lead paint to defective tires to tainted toothpaste and food. Despite the high-profile problems, America's trade deficit with China was running at record-high levels.

6. GLOBAL WARMING: Warnings about the consequences of global warming gained intensity with new reports from scientific panels and a Nobel Prize to Al Gore for his environmental crusading that included the film "An Inconvenient Truth." Across the U.S., many state governments sought to cap emissions blamed for global warming.

7. BRIDGE COLLAPSE: An Interstate 35 bridge over the Mississippi River in Minneapolis collapsed during the evening rush hour on Aug. 1, killing 13 people and injuring about 100. The disaster fueled concern about possible structural flaws in other bridges nationwide.

8. PRESIDENTIAL CAMPAIGN: In a yearlong drama with shifting subplots, large fields in both major parties battled for support ahead of the caucuses and primaries that will decide the 2008 presidential nominees. Hillary Clinton and Barack Obama led among the Democrats; some polls showed five Republicans with double-digit support.

9. IMMIGRATION DEBATE: A compromise immigration plan, backed by President Bush and Democratic leaders, collapsed in Congress due to Republican opposition. The plan would have enabled millions of illegal immigrants to move toward citizenship, while also bolstering border security. The issues remained alive in the presidential campaign.

10. IRAN'S NUCLEAR PROGRAM: Worried that the ultimate goal is a nuclear arsenal, the United States and other countries pressed Iran to halt uranium enrichment. Iran said it never had a weapons program. A U.S. intelligence report concluded there was such an effort, but it stopped in 2003.