Most of us love a good conspiracy or two - from ancient Rome to Watergate, conspiracies are as old as the hills, cropping up time and again throughout history. That they emerge with such frequency is hardly surprising - conspiracies, after all, are simply when two or more people agree to perform an illegal, wrongful or subversive act, and if there's one thing people like to do it's break the law for their own advantage. Equally unsurprising is the number of times conspiracies have formed the basis for stories in movies - a good conspiracy by definition contains all the elements of a decent plot, from the conspirators themselves and their motives to those who would seek to expose them and the unfortunate bystanders caught in the middle. It's a perfect template for an engrossing and head-scratching thriller, challenging the audience to unravel the plot before the credits roll. Some of the best conspiracy movies often use real-world conspiracies - both theoretical and proven - on which to base their storylines, whether it's deception and fraud on the part of the political classes designed to justify immoral policies or corporations colluding in secret to hide damning evidence from the public. Their relevance to reality adds to the feeling of authenticity and reminds us that sometimes the truth is stranger than fiction. The following list covers ten of the greatest conspiracy movies ever made - films in which nothing is what it seems and everyone has hidden motives bubbling beneath the surface.