ABSTRACT Problem-solving is one of the necessities of life in twenty first century. Therefore, Psychologists consider it as a skill that everyone must learn it. The purpose of the present study is to compare the effectiveness of the transactional analysis, existential, cognitive, and integrated group therapies on improving problem–solving skills. For this purpose, 65 subjects of the clients who were referring to the Ferdowsi University of Mashhad\\\\\\\\\\\\\\\'s Mental Health Centre were selected randomly and subjects placed in 5 groups in random assignment method (13 participants in each group). The research method is Pre-test/Post-test control group design. To gather the data, Long & Cassidy’s problem solving styles questionnaire (1996) was used. In the descriptive level, the data were analyzed using mean and standard deviation, and in the inferential level Analysis of Covariance test (ANCOVA) was used. The results of data analysis were indicative of the fact that after modifying pretest scores, there was a significant difference between group’s subjects. The results showed that group therapies were effective on improving problem-solving skills and that cognitive and integrated group therapies were more effective on improving problem-solving skills comparing to other groups. Regarding the results of the present study, it can be concluded that transactional analysis, existential, cognitive and integrated group therapies were effective on improving problem-solving skills of the clients who were referring to the Ferdowsi University of Mashhad\\\\\\\\\\\\\\\'s Mental Health Centre.

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