Newsbeat guide: What's the Leveson Inquiry all about?

The Leveson Inquiry was set up by the prime minister after some journalists admitted illegally listening to people's voicemails.

Celebrities, sporting figures, and MPs had their phones hacked - as well as crime victims like the murdered schoolgirl Milly Dowler.

It's looking much more broadly though, at relationships between the press, politicians and other people in power.

It will end up recommending new rules on how media organisations should work.

Sienna Miller, Hugh Grant, Steve Coogan and the Harry Potter author J K Rowling are among the many celebrity witnesses.

Murdered Milly Dowler's mother and father gave evidence, as did Kate and Gerry McCann, the parents of missing Madeleine.

They all claim they've been victims of media wrong-doing.

Former Prime Ministers Tony Blair, Gordon Brown and John Major all took part - and today (14 June) the current PM David Cameron takes the stand for a whole day.

David Cameron may be dismayed that the inquiry he set up has focused so often on his own friendships with powerful media people.

There's been lots of interest in his dealings with Rupert Murdoch - who shut down the News of the World over phone-hacking but who still owns the Sun and a large chunk of Sky TV.

It's come out previously that the Prime Minister borrowed a horse from ex-News of the World boss Rebekah Brooks and signed off texts to her with the letters L O L - which he took to mean 'lots of love' instead of 'laugh out loud.'

David Cameron gave the job of assessing a Murdoch bid for Sky TV to Culture Secretary Jeremy Hunt, who'd previously been described as a 'cheerleader' for the company.

Elsewhere he brought another ex- Murdoch editor into his Downing Street team who's now facing a criminal charge.

If he escapes without creating new headlines about any of the above - he'll call his day of questions a success.

Lord Justice Leveson says he'll publish a report within a year.

It will make recommendations on how media organisations should be policed, and how politicians, police and journalists should behave around each other.