The 60-item Multidimensional Psychological Flexibility Inventory (MPFI) was created within the Hexaflex conceptual framework of acceptance and commitment therapy (ACT). The MPFI assesses the six dimensions of psychological inflexibility targeted in ACT and the six corresponding dimensions of psychological flexibility promoted in ACT by means of 12 distinct 5-item subscales. A shorter 24-item version of the scale has also been developed and validated. Notably, recent work on the MPFI has highlighted how the dimensions it assesses can also be conceptualized as a diverse and expanded set of mindfulness processes aligned with meditation practice and key tenets of Buddhism, thereby offering mindfulness researchers and clinicians a powerful tool for assessing and operationalizing 12 distinct mindfulness processes. The MPFI was developed from a pool of 554 items across three large-scale online samples (ntotal = 3040). Items were selecting using exploratory factor analyses (EFAs) and item response theory (IRT) analyses to optimize the information provided. The final set of subscales demonstrated a stable factor structure across random sample halves, excellent levels of internal consistency that generalized to 18 distinct demographic populations, convergent validity with existing scales, discriminant validity with one another, unique predictive validity exceeding existing scales, and higher levels of information (lower measurement noise) than existing scales. The MPFI subscales also offer the possibility of more nuanced findings both in basic research and clinical settings. To date, validated Spanish, French, Italian, Swedish, Portuguese, Farsi, Persian, simplified Chinese, traditional Chinese, Japanese, and Korean translations of the MPFI have been published.