Mixed research designs in psychotherapy combine qualitative and quantitative methods to evaluate clinical change, allowing a comprehensive view of psychotherapeutic processes. Throughout the history of psychotherapy research, despite the development of mixed designs, the idea has persisted that quantitative and qualitative methodologies are incompatible fields of knowledge production, arguing that they respond to different epistemological positions. However, as this chapter outlines, an integrative approach proposes articulating both methods coherently to enrich the understanding of clinical phenomena and the psychotherapeutic process. A classification of mixed designs is presented, including temporal and hierarchical relationships in integrating both methods. Together, this variety of designs offers a richer and more detailed view, capturing both the participants' subjective experience in the psychotherapy processes and observable data that theoretically validate the interventions that occur in this space and contribute to psychotherapeutic change. Each of these types of mixed designs is exemplified with research conducted in the region and by researchers from other parts of the world to expose the rationale that allows understanding of the relevance of the use of these models. This is considered the object of study of clinical phenomena and psychotherapy.