With the increased application of mindfulness-based interventions, the evaluation of their effectiveness requires more accurate measurement of both mindfulness and related outcomes. In particular, issues to do with reliability and validity, measurement precision (e.g., ordinal versus interval scaling), individual item functioning, and the distinction between dynamic (state) and enduring (trait) aspects of mindfulness have not been sufficiently addressed or resolved using appropriate statistical methods. Ordinal measures have limited precision and using them with parametric statistical techniques violates the fundamental assumptions of these tests. The accurate distinction of state from trait mindfulness and establishing measurement at an interval level are two essential steps for rigorously validating mindfulness and outcomes measures. Modern psychometric methods provide solutions to evaluate and improve the scaling properties of ordinal measures and distinguish quantitatively between state and trait components in a measure. This chapter introduces current issues and developments in mindfulness assessment along with outlining the structure of the present handbook, which aims to provide the reader with all the necessary background to make an informed decision about measurement of mindfulness and other relevant outcomes used in mindfulness research.