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It's official: An asteroid wiped out the dinosaurs
A giant asteroid smashing into Earth is the only plausible explanation for the extinction of the dinosaurs, a global scientific team said on Thursday, hoping to settle a row that has divided experts for decades.

From an article, The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous-Paleogene Boundary, in the Journal Science just this week.

The Cretaceous-Paleogene boundary ~65.5 million years ago marks one of the three largest mass extinctions in the past 500 million years. The extinction event coincided with a large asteroid impact at Chicxulub, Mexico, and occurred within the time of Deccan flood basalt volcanism in India. Here, we synthesize records of the global stratigraphy across this boundary to assess the proposed causes of the mass extinction. Notably, a single ejecta-rich deposit compositionally linked to the Chicxulub impact is globally distributed at the Cretaceous-Paleogene boundary. The temporal match between the ejecta layer and the onset of the extinctions and the agreement of ecological patterns in the fossil record with modeled environmental perturbations (for example, darkness and cooling) lead us to conclude that the Chicxulub impact triggered the mass extinction.

When I was a kid there were literally dozens of theories for what killed the dinosaurs (from poisonous plants to overheated testicles). In 1980 an asteroid impact hypothesis got a big boost by the discovery of Iridium in the sedimentary rocks at the K-T boundary. In the 30 years since then this theory has gone from being highly speculative to being mentioned in textbooks to being generally accepted and finally it has become scientific orthodoxy.