Psychological flexibility (PF), and its psychopathological reflection psychological inflexibility (PI), are the main purported processes of change within acceptance and commitment therapy (ACT). A large body of evidence exists supporting the distinctiveness of these two constructs (along with the 12 sub-processes within the hexaflex model of PF and PI) and their relations with relevant mental health-related outcomes. For researchers and clinicians alike, clear operationalization of both processes of change and outcomes in ACT– along with psychometrically sound measures for assessing them– is needed to rigorously evaluate its efficacy. Several challenges have arisen throughout the history of ACT in terms of measuring PF and PI, including psychometric limitations of existing measures such as the Acceptance and Action Questionnaire series, a lack of context specificity in available measures, and overreliance on retrospective self-report of trait-based constructs. However, several recent psychometric innovations hold promise for addressing these concerns. In this chapter, we provide an overview of the state of the evidence for existing measures of PF and PI, interwoven with clinical implications. This is followed by a discussion of specific recommendations and practical tips for measuring PF and PI in clinical practice. Our main suggestions include: (a) keep science-informed measurement top of mind, (b) remember the truth criterion of workability within assessment, and (c) intellectual humility, amiable skepticism, and intense curiosity can facilitate reliable and valid measurement of PF and PI. Application of rigorous measurement principles to assessment of PF and PI in clinical practice is likely to further improve outcomes for clients.