It could be anything from a simple 'hello' to disrupting the earth's nuclear fusion process.
But despite the uncertainty of how aliens could get in touch, one thing is certain for the astronomers from Search for Extraterrestrial Intelligence - there would not be a cover-up.
The small band of researchers around the world scan the universe for signals coming from the world's largest radio telescopes, hunting for any unusual signs.
And while conspiracy theorists argue there would be a government cover-up or global upheaval Seth Shostak, the Seti Institute's principal astronomer, told the BBC that both groups are unnecessarily panicking.
'The idea that governments would keep this quiet because otherwise the public would go nuts, is nuts. History shows that's not what happens.
'In the early 1900s, there were claims that there were canals on Mars - a vast hydraulic civilisation just 50m km from Earth. The average guy in the street said 'well, I guess there are Martians' - they didn't panic.'
But no one would believe them until it was on CNN or Fox News, and that's unlikely to happen. To read more, click here.