From the perspective of research on the process of change in psychotherapy, we used two main constructs, in a complementary way, to understand how change occurs in psychotherapy. One of these constructs is part of the approach of significant events developed by Elliott (1985) defining "significant event" as any moment located in therapy sessions, where the client experience at some level, their change. These moments consist of segments of therapy sessions, which refer to some interaction, action or specific reaction, and that involve a helpful therapeutic effect for the client improvement. Significant events research has been developed in order to understand the most productive moments of the therapeutic work and to guide the therapist’s actions. In addition, this approach assumes that these events cause helpful impacts that reflect the steps or processes by which the therapy produces the changes. The selection of the second studied construct is based on the assumption that the change in psychotherapy is through a good relationship between the therapist and client. A good alliance and a collaborative work between the dyad are often associated as predictors of good therapeutic outcomes. Furthermore, studies investigating the process of change consider interesting to understand how the therapeutic relationship contribute to change in therapy, and has been focused in the interactive microprocesses involved in the development of the relationship between therapist and client. Thus, in this thesis we assume the construct of the therapeutic collaboration, analyzed a microanalytical level, also as central. Several empirical studies reviewed in systematic review of literature – our first study, pointed to a lack in the literature on the process of change in the approach of significant events and the therapeutic relationship simultaneously. We consider that to understand the processes of change would be interesting to find out what happens in the dialectical working moment-to-moment between therapist and client and possible relationships with significant events and helpful impacts. However, our prior aim was to analyze the significant events and the helpful impacts, considering the complementarity of the client and therapist perspectives. We developed a System of helpful impact categories in the second study, which is included in this thesis. This system was applied in the third study, when we compared a good and a poor outcome cases and in the fourth study, when we relate the events and therapeutic exchanges. Using an inductive methodology, we categorize the events differentiating them according to three categories: the therapist actions, client actions, or joint actions. Using a deductive methodology from the system of helpful impact categories we categorized the helpful impact on the client’s change. In general, the results were consistent with the theory and it does not seem to be a pettern that differentiates the two perspectives, although it was possible to see a greater divergence between the client and therapist perspectives in the poor outcome case than good toucome case. In the fourth study we used the Therapeutic collaboration coding system in order to microanalyze, in a moment-to-moment level, the therapeutic exchanges among the dyad at the start of significant events. We analyzed the significant events using an inductive method and categorized the helpful impacts from the System of helpful impact categories. The results of this study suggest that interventions on the client level of potential development are very important in the promotion of significant events, as well as the safety or intolerable risk client experiences. This suggests that interventions outside the client development zone can also be significant, contributing to the process of change in psychotherapy if it occurs in a therapeutic climate characterized by safety and comfort.