The commitment to evidence-based practice in clinical psychology requires scientific investigation of the effects of treatment and mechanisms of change. Empirical evidence suggests that mindfulness-based treatments provide clinically meaningful improvement for people suffering from many important problems, including depression, anxiety, pain, and stress. However, the processes of change that produce these beneficial outcomes are not entirely clear. Central questions include whether mindfulness training leads to increases in the general tendency to respond mindfully to the experiences of daily life, and if (...) so, whether these changes are responsible for the improvements in mental health that are often observed. Answering these questions requires methods for assessing mindfulness. Several tools for this purpose are now available and early evidence suggests that they are useful and informative, despite shortcomings that require additional work. This paper summarizes the rationale for mindfulness questionnaires, the methods used to construct them, and research findings on their utility. Challenges related to maintaining consistency with Buddhist conceptions of mindfulness while applying psychological research methods to the study of its assessment are discussed. (shrink)