ABSTRACT Theorizing has focused on individuals' self-representations as a psychological resource for coping with life stress and developmental challenges in adulthood. Many of the prominent theories have conceptualized self-representations with regard to specific social contexts (e.g., role-specific self-representations) and have examined specific structural organizations of the self-concept with regard to psychological adjustment. This article describes research on the associations between self-concept structures and psychological well-being in adulthood. Specific emphasis is given to the feature of self-concept differentiation (SCD). Most research suggests that a high level of SCD tends to indicate self-fragmentation and tends to be associated with poorer adjustment and psychological well-being. Findings from a daily diary study with adults of all ages are reported showing that different levels of SCD were in a consistent and meaningful way related to the daily endorsement of positive and negative self-attributes. Daily self-representations, in turn, were significantly related to individuals' level of daily negative affect and to intra-individual variation in negative affect. These findings suggest that SCD may exert its effect on adjustment and psychological well-being through specific ways of processing self-related information.