Ukraine's parliament has opened a special session dedicated to the accident at the Chernobyl nuclear plant 20 years ago today.
Hundreds of people are gathered in Chernobyl.
They're about ten miles from the plant where Reactor Number Four exploded in the pre-dawn hours of April 26, 1986.
President Viktor Yushchenko was on hand.
He flew over the crumbling concrete-and-steel shell that covers the damaged reactor.
Commemorative ceremonies are also being held in Kiev.
In Vatican City, Pope Benedict recalled the world's worst nuclear accident and urged world leaders to ensure that all forms of energy are used peacefully in the service of humanity.
About four thousand people still work in the most highly contaminated zone at the plant, but they are allowed there for no more than two weeks at a time.