We do not know much about how patients internalize, remember or use psychotherapeutic experiences between psychotherapy sessions and how they use them for change. What happens with in-session-experiences after a session ("inter-session-process") is of main importance for outcome. The inter-session-process describes the work on psychotherapy between sessions. Although inter-session-processes are a central element connecting psychotherapy process and outcome, they remain a neglected area of research. Previous studies examined individual outpatient settings only. This paper gives an overview over theory, instruments for measurement, previous studies and relevance of inter-session-processes for the field of psychotherapy research.