One of the goals of research in the field of psychotherapy is to improve knowledge about processes and outcomes of psychotherapeutic treatments. Researchers and professionals have been discussing the best methods for evaluating the effectiveness and efficiency of psychotherapeutic treatments for many years. This paper aims to give an overview of the specifics of quantitative and qualitative research methods, by noting the advantages and disadvantages of these methods in the evaluation of psychotherapeutic treatments. Within the quantitative scientific research, three approaches are described: randomized controlled research, mood enhancement by psychoanalytic and cognitive therapies, and meta-analysis. The most common collection methods (observation, interview, other verbal techniques and visual approaches to data collection) and data analysis (comprehensive process analysis, consensual qualitative research and grounded theory) are described within a qualitative scientific methodology. Finally, an approach related to integration of qualitative and quantitative methodology, as well as this related with application of case studies in the evaluation of psychotherapeutic treatments are described. Scientists and professionals in the field of social sciences should use both quantitative and qualitative research methods, separately or in combination, depending on the goal and problems of the research.