This chapter presents the range of techniques available to assess mindfulness as well as the approaches that have been used to test their reliability and validity. A large number of mindfulness self-report instruments are available, which allows researchers to focus on different aspects and nuances of mindfulness, such as present-moment awareness or aspects that are taught in some specific mindfulness-based interventions. The psychometric properties of mindfulness questionnaires have become tested using increasingly sophisticated approaches that permit scrutinisation of item performance as well as differentiation between state and trait mindfulness. A major disadvantage of self-report measures of mindfulness remains the issue of subjective reference standards that may shift with mindfulness practice. This issue of response shift has started to be studied in the literature, but remains to be explored in more detail. While self-report instruments remain the most common way to assess levels of mindfulness, a number of performance or behavioural measures have been proposed that involve recording the awareness of breath and thus may be less affected by response biases. The chapter ends with a brief discussion on how measurement of mindfulness has the potential to guide further conceptual development of the construct.