Philosophers have always been interested in conspiracies, but the philosophical debate on conspiracy theories is a pretty recent phenomenon. Karl Popper wrote about the ‘conspiracy theory of society’, but his discussion concerned issues of intentional explanations in general rather than conspiracy theories proper. This chapter explains how philosophy could help studying conspiracy theories. It focuses on approaches that represent ‘analytical’ philosophy rather than ‘continental’ philosophy. Examples of conspiracy theories include claims that deal with issues such as the death of Princess Diana, the origin of A.I.D.S. and the truth of 9/11. The concept of conspiracy theory is commonly used in a pejorative sense. Philosophers disagree about whether conspiracy theories should be distinguished from ordinary social explanations that refer to conspiracies. Some philosophers have suggested that all the explanations that refer to conspiracies should be called ‘conspiracy theories’ Conspiracy theorising is a form of human action and is thus an appropriate object of ethical evaluation.