The literature review critically evaluated research articles focusing on deterioration in psychotherapy published since a watershed review by Mohr (1995). This review adopted the recommendations made by Mohr (1995) as a framework for the literature. A total of 28 studies were identified and reviewed using a quality rating system derived from Mohr's recommendations according to the extent to which these recommendations were implemented in the identified studies. The review yielded a higher average rate of deterioration (9-17%) in comparison with Mohr's review (5-10%). It was concluded that research into deterioration generally has continued to suffer from methodological limitations. The intention of the research report was to investigate the phenomena of overall deterioration and sudden deterioration in a routinely collected data set collected from the Improving Access to Psychological Therapies (IAPT) initiative. Sudden deterioration was explored to determine whether it existed and how it may be defined. The rates of deterioration within the IAPT data were identified, and predictors of these were assessed. It was determined that an appropriate definition for sudden deterioration was a reliable between-session change using the Patient Health Questionnaire-9; PHQ-9), that was not allied to sudden gains. Rates of sudden deterioration and overall deterioration were found to be 3.4% and 3.1% respectively. It was concluded that sudden deterioration exists as a phenomenon, is closely related to overall deterioration and that rates of deterioration in the IAPT dataset were relatively low.