This paper is a study of Conspiracy Theory, the theory according to which the causes which explain the occurrence of many events and phenomena are not the officially advertised causes which the public media present us with; rather, the events and phenomena in question should be viewed as the work of agents and agencies that operate in secret in the service of projects that may or not be publicly known. The paper discusses the relation between theory and evidence and compares conspiracy theories to others, especially scientific theories. Two conclusions emerge. Firstly, a theory should not be rejected merely on the ground that it is a conspiracy theory. Secondly, most conspiracy theories do not deserve to be taken seriously because of the consequences which follow from their method of dealing with counter evidence.