This is still a controversial question. One of the leading theories is that the earth was struck by a meteorite of considerable size, and this impact threw up immense amounts of dust and other particulates into the atmosphere. These particles changed the atmosphere so that a greater percentage of sunlight was reflected back into space, causing global cooling. Because this happened quickly, species were unable to adapt and many died off.

Several other theories also depend on climate change, but cite different possible reasons for the changing climate: continental drift, volcanoes changing the composition of the atmosphere, or changes in the earth's orbit have all been suggested.

Another theory is that mammals were actually responsible because they ate the dinosaur's eggs.

Why did dinosaurs go extinct?

While there are no definite answers to this question, the leading theory has to do with an asteroid strike off the coast of the Yucatan Peninsula in Mexico. This crater off the coast of Mexico has been dated to 65 million years, so it coincides approximately with the time of the last dinosaurs of the Cretaceous period. This killed the dinosaurs in the immediate vicinity of the crater due to the tidal wave and the boiling water brought forth by the asteroid. The water vapor and dust put forth in the atmosphere would have blocked out the sun and killed most of the plants at the bottom of the food chain, as well as much marine life. The temperature drop would have killed the dinosaurs as well, as they would not have been able to go for cover unlike other cold-blooded smaller reptiles and amphibians. Another theory has to do with increased volcanic activity around the Indian subcontinent. This plate boundary is still quite active today. With either scenario, scientists point to climate change as being the leading cause.  

Why did dinosaurs go extinct?

Dinosaurs flourished and dominated the earth for about 186 million years, during the Mesozoic Era. Dinosaurs went extinct at the end of the Mesozoic era and at the beginning of the Paleogene period, the first period of the Cenozoic Era. About 65.5 million years ago, a giant asteroid hit the earth in Mexico's Yucatan Peninsula. It is estimated that the asteroid had a 10 km diameter and was about the size of a small town. The asteroid hit our planet so strong that, after the impact, which was 2 million times more powerful than the most powerful explosive ever detonated by humans, it formed the Chicxulub crater. The ambient temperature immediately rose in some areas to as high as 1,480 degrees Celsius, which led to the burning of forests and of all things unprotected.