Research in psychotherapy and clinical psychology is rapidly expanding. Over time, alongside clinical studies, theoretical contributions, and studies based on multivariate analysis, increasingly cutting-edge research designs have been developed: multilevel studies, use of differential equations, network analysis. The latter are based on dynamic systems, that is, on the study of temporal changes of  process variables of a given system. This systemic approach is the ground on which this special issue is based. Clinical, theoretical, methodological contributions, studies based on multivariate analysis, based on differential equations, multilevel studies, network analysis are taken into consideration, provided that the main focus of the work is on temporal evolutions of process variables of the system-at-hand. Understanding the dynamics of change of patients is the key ingredient to promote positive treatment outcomes.