The purpose of this chapter is to provide the theoretical foundation, development, validation, and guide for using the Unconditional Self-Kindness (USK) Scale in mindfulness research. The USK is based on the Pali word “metta” which has been translated in English as “loving kindness.” Specifically, the USK is designed to assess self-kindness or loving kindness toward the self without conditions. The USK includes items to assess both active and passive forms of self-kindness in relation to threats to the self in the domains of achievement, interpersonal relationships, and personal characteristics. This chapter presents validation data showing the USK and its subscales are both reliable and valid. The reliability includes both internal consistency and test–retest reliability. The validity includes consistent positive correlations with protective factors, consistent negative correlations with risk factors, and incremental validity in predicting good mental health outcomes controlling for other important predictors of better mental health. Finally, the chapter presents the items and guidelines and suggestions for using the scale in mindfulness research.