The Acceptance and Action Questionnaire-II (AAQ-II) is a widely used tool for assessing psychological inflexibility. Developed to address limitations of the original AAQ, this 7-item measure has been validated across multiple languages and contexts, making it applicable across a wide variety of settings. The AAQ-II demonstrates robust psychometric properties, including strong reliability and validity across diverse samples. It is frequently utilized in both research and clinical settings due to its versatility, ease of administration, and brevity. Despite some ongoing ambiguity regarding its discriminant validity, particularly its overlap with measures of general distress, the AAQ-II remains an invaluable instrument in mindfulness and meditation research. It plays an important role in guiding treatment interventions and monitoring treatment outcomes, especially within Acceptance and Commitment Therapy (ACT) and mindfulness-based approaches. The measure’s numerous context-specific adaptations further enhance its utility across various problem areas. However, newer measures have been developed to address some limitations of the AAQ-II, providing more nuanced assessments of psychological inflexibility.