The End-Cretaceous extinction
Scientists are researching many different factors that may have played a
role in the extinction of the dinosaurs at what is called the K-T boundary
(Cretaceous-Tertiary boundary), which occurred 65 million years ago.
Meteorite impact.
There is a lot of evidence of this, including:

discovery of a large buried crater at Chicxulub in what is now
Mexico. This crater is several hundred kilometers across and would
have been made a meteorite nearly 10 km across travelling at
thousands of miles per hour.

A thin layer of clay, containing large amounts of an element called
iridium (which is rare on Earth, but abundant in meteorites), has now
been found at many sites all around the world at the K-T boundary.
The most likely source for this iridium is a meteorite.

There is also evidence of a giant tsunami (tidal wave) caused by the
impact in North and South America and the Caribbean and some
scientists even think that they have found part of the meteorite
itself.
As well as killing those animals close to the impact, the impact would have
had global consequences: throwing millions of tonnes of debris into the
atmosphere, blocking out sunlight, stopping plant growth for several years
and cooling the climate. Plant-eating animals would have died out first,
followed by meat-eaters.
Eruption of huge amounts of lava
There is evidence for a set of massive eruptions at this time. In particular,
there is an area of central India, known as Deccan, which shows evidence
of huge lava flows of the right age. These are called the 'Deccan traps' and
are thought to have resulted from extensive volcanic activity lasting around
1 million years, with the activity happening in fits and starts, with gaps in
between separate major lava flows. Millions of cubic miles of lava erupted
during this time.
Such large eruptions would have killed animals in the immediate area but,
more importantly, would have also had a major effect on global climate
due to the release of huge amounts of ash and greenhouse gases (H20,
CO2 and SO2, in particular). These climate changes are suggested to have
caused ecosystem collapse that killed the dinosaurs.
Gradual climate change
The Cretaceous period was generally the warmest ever period in the
history of the Earth, but there is evidence of it cooling toward the K-Tboundary. During the Cretaceous the dinosaurs the continents were closer
together and more joined up than they are today. During the end of the
Cretaceous Period, the continents were drifting apart, and becoming more
like what we are familiar with today. This caused changes in ocean currents
that led to a general global cooling. This may have contributed to the
extinction.
A combination of factors?
It’s very likely a combination of factors killed the dinosaurs. The dinosaurs
were not the only animals to die out at the end of the Cretaceous, many
groups of reptiles, invertebrates and others did as well. Approximately 65%
of species living at the end of the Cretaceous went extinct. Interestingly,
nothing over 25 kg survived and animals living in freshwater were not as
badly affected as those in the sea or on the land.
Animals living in freshwater have various advantages over those on land as
water is less subject to temperature changes than air and their ecologies
are based much more on access to dead matter than those on land, which
are more reliant on plants. Freshwater also has advantages over sea water
as oxygen levels higher in freshwater than saltwater and again, there is
more dead matter in freshwater than seawater. These observations suggest
that all of the ecosystems were collapsing and couldn't support large
animals.
Further research
http://web.ukonline.co.uk/a.buckley/dino.htm
http://www.ucmp.berkeley.edu/diapsids/extinction.html
http://palaeo.gly.bris.ac.uk/Communication/Couch/101Theories.html
http://en.wikipedia.org/wiki/Dinosaur_extinction