This monograph presents a new analysis of the 'straight' self-deception that depends upon motivational bias as well as an account of the unstudied phenomenon of self-deception that is dependent on expectational bias. Cases of 'twisted' self deception are then explained as resulting from a conflict between motivational and expectational bias. In all cases, self-deception is shown to be an unintended result of an agent's intentional activity directed at saving a theory toward which she is biased. The exposition is novel in drawing on insights from the philosophy and history of science to illuminate the problem of self-deception. The central notion in the new analysis is that of an ad hoc manoeuvre, which is taken from Karl Popper. The analysis of self-deception is supported by various new examples, the most detailed of which is a case study of a self-deceived homosexual, which is taken from the author's own life and which gives an intimate insight into the dynamics of the phenomenon. Throughout, the approach taken by the author is critically compared with the best of the other available accounts of self-deception. This book is intended as a clear and readable introduction to the murky topic of self-deception that is accessible to the layperson as well as undergraduates, while also providing stimulation, insight and provocation to the academic philosophers and psychologists doing research in this area.