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As non-governmental organisations descend on Rio for next week's Earth Summit, they bring with them environmental activists from around the world, pressing for attention for a huge range of issues.

This week on One Planet we're looking at the role of the activist. Who are they? What do they do? What can they really achieve? And where do they draw the line?

We speak to people from all over the activist spectrum, from celebrities to environmental campaigners to protesters who advocate breaking the law in support of their cause.

We visit the new Occupy encampment on Hampstead Heath in London; talk to campaigner Mel Evans about whether direct action is the best way to get an issue noticed; and speak to environmentalist Lo Sze Ping about the challenges of activism in China.

Plus, campaigning journalist Andrew Revkin tells what happens when reporters join the debates they're meant to be covering; and we ask celebrity campaigner Bianca Jagger why famous faces so often want to attach themselves to a cause.

As ever, have a listen and let us know what you think. You can email the team at oneplanet@bbc.com, follow us on Twitter, or join us on Facebook - the links are below.