This research details a series of questionnaire and experimental studies investigating the structure of unassertiveness and the link between unassertiveness and depression. Seven studies with sample sizes greater than 100, were carried out. Study one focused on the design and validation of a questionnaire to measure understanding of assertive rights and assertive responsibilities. Test retest, reliability, split half reliability, internal consistency, construct validity and content validity were found to be adequate. Study two validated an amended version of the above scale. Changes in scoring method resulted in increased reliability and validity. Study three replicated and expanded Chan's (1993) study and investigated the construct of assertion in a British population, the relationship between assertiveness and assertion related rights and responsibilities and how all of these factors related to emotional disorder. Levels of understanding of assertiveness rights and assertiveness responsibilities were correlated with depression, state and trait anxiety. A step-wise regression revealed the four factors of assertion related to high levels of depression and high levels of state and trait anxiety. Study four determined the factors of depression and their predictive ability of a lack of assertiveness. Depression was factor analysed and a step wide regression demonstrated that only cognitive factors were predictive of a lack of assertiveness. Study five focused on the design and psychometric testing of a scale to measure the presence of cognitive distortions. The data collected was used to demonstrate the same forms of validity and reliability found in study one and two. Study six considered the possibility that the cognitive components of depression proposed in the literature are also present in those who lack assertiveness. Both experimental and questionnaire techniques are implemented. Stroop testing was utilised to assess attention biasing, computerised free recall memory tasks were used to assess memory biasing and cognitive depressive aetiology were assess by questionnaire. Comparisons were made between a high depression group, a low depression group a high assertion group and a low assertion group. The results demonstrate that episodes of unassertiveness are generated by an interaction between continual unassertive specific life experiences and a critical unassertive related incident. The continual presentation of negative assertion related incidents throughout life generates the activation of dysfunctional assertion related attitudes, which in turn generate and are maintained by a negative cognitive triad. Study 7 is a clinical trial, which looked at the components of study 6 in a population of 30 individuals who had been diagnosed unassertive by a clinician and were participating in an assertion training class. In this study the participants were tested before and after an assertion training course, to assess the changes that occur as a result. of assertion training. These finding corroborate that the factors found in study six generalise to a clinical population. These studies have highlighted that unassertiveness has a greater cognitive emphasis than previously thought. As a consequence of these findings a cognitive model of unassertiveness is presented.