This chapter examines the Freiburg Mindfulness Inventory (FMI), a landmark instrument that holds historical significance as the first formally developed mindfulness measurement scale. We trace the development, validation, and evolution of this pioneering measure, which emerged at a crucial time when scientific interest in mindfulness was gaining momentum but standardized assessment tools were lacking. The chapter details how the FMI was initially conceptualized and validated with insight meditation (Vipassana) practitioners, leading to its original 30-item version and subsequent refinement to a 14-item scale. We explore the inventory’s distinctive theoretical foundations in Buddhist contemplative traditions and discuss its psychometric properties across different populations. Special attention is given to the instrument’s unique characteristic of requiring prior mindfulness experience for valid assessment, which has important implications for its application. The chapter also reviews international adaptations of the FMI, including translations into French, Chinese, Dutch, and Finnish, examining their validation processes and cultural considerations. Through comprehensive analysis of empirical studies, we address both the strengths and limitations of the FMI, particularly focusing on response shift effects and item comprehension among different user groups. This chapter is intended to provide researchers and practitioners with essential knowledge for selecting appropriate mindfulness assessment tools and understanding the historical context of mindfulness measurement in psychological research.