This thesis details the construction of a scale to measure beliefs that people have about successful relationships - the Relationship Success Belief Scale. In Study 1, 100 respondents provided a pool of statements from which 18 basic factors for relationship success were derived. The scale was constructed, and pilot tested on 287 subjects. Study 2 conducted with 451 respondents established that the scale can be used across different relationship types and genders, and showed that the scale has adequate internal and test-retest reliability. The factor analysis revealed a meaningful five-factor structure. Study 3 examined the convergent and discriminant validity of the five sub-scales. As expected, relationship success beliefs were not related to social desirability or relationship satisfaction, but aspects of the scale were positively associated with a secure attachment style and certain attitudes about love. Evidence for the predictive validity of the scale was provided in Studies 4 and 5. In Study 4 a focus on the relationship, as opposed to the self or the partner, was associated with harmonious beliefs. Study 5 showed that specific relationship behaviours and relationship satisfaction are more closely linked in people who have strong beliefs in these areas, than in those who have weaker beliefs. Clinical implications of the scale are examined and suggestions for future research into the psychometric properties of the scale are outlined. In addition, a research programme exploring the linkage between relationship success beliefs and other elements of a contextual model of relationships is detailed.