Over-valued ideas are truly a neglected area of psychopathology with few experimental studies published. There is a different emphasis in the USA and Europe regarding their definition. For authors in the USA an over-valued idea has become shorthand for 'poor insight' in the middle of a continuum of obsessional doubts to delusional certainty. Compared to negative thoughts, obsessions and delusions, they are often more resistant to any treatment. A better understanding of over-valued ideas is required if advances are to be made in therapy and for the development of appropriate measures to evaluate the efficacy of novel treatments. A cognitive behavioural model of over-valued ideas is presented which draws upon the philosophical distinction between beliefs and values. It is argued that over-valued ideas are associated with idealised values, which have developed into such an over-riding importance, that they totally define the 'self' or identity of the individual. Idealised values are also characterised by the rigidity with which they are held. Such patients are unable to adapt to different circumstances and ignore the consequences of acting on their value. This analysis leads to a discussion of predictions that can be tested and various strategies that can be used in cognitive behaviour therapy.