Why has Hosni Mubarak stepped down now?

News that Hosni Mubarak has stepped down as president of Egypt, after weeks of protest across the country has been greeted with a huge outburst of joy by thousands in Cairo's Tahrir Square.

Egypt is effectively under military rule and the generals hold the key to what happens next.

They have said they would bring an end to decades of emergency law.

The BBC's Jon Simpson reports on Egypt's future.