Assessing mindfulness is vital for evaluating the effectiveness of mindfulness-based interventions. This chapter synthesizes the conceptual foundations, development, and psychometric properties of key mindfulness measures. It addresses issues related to reliability, validity, measurement precision, and the distinction between state and trait aspects of mindfulness. Modern psychometric methods offer solutions to evaluate and improve the scaling properties of ordinal measures and quantitatively distinguish between state and trait components. This chapter introduces current issues and developments in mindfulness assessment, providing readers with the necessary background to make informed decisions about measuring mindfulness and other relevant outcomes in mindfulness research.