Buddhist thought and contemporary psychological science suggest that an important mechanism through which mindfulness contributes to well-being is by reducing self-referential processing of experience and cultivating selfless processing of experience. Self-referential processing of experience is implicated in prevalent forms of suffering and mental health problems. In contrast, selfless processing of experience is considered to be fundamental for the cultivation of attitudinal qualities of mindfulness (e.g., non-judging) and of a metacognitive process subserving decentering (e.g., disidentification from internal experience). Critically, study of these processes has been limited by the absence of behavioral measurement methods of self-referential and selfless processing of experience. Accordingly, the Single Experience and Self-Implicit Association Test (SES-IAT) was developed to behaviorally measure these processes. It involves the experimental elicitation of a subjective experience (e.g., elicitation of fear using videos and audio), while concurrently measuring the cognitive association between self and the elicited experience (e.g., fear) by means of a Single Category-Implicit Association Test. Based on this method, the fear SES-IAT was designed to measure self-referential and selfless processing of fear. Experimental and correlational findings indicate that the fear SES-IAT measures selfless processing of fear, identification with fear, and negative self-referential evaluation of fear. Importantly, findings also support the fear SES-IAT as a measure of key attitudinal qualities of mindfulness and of a metacognitive process subserving decentering (i.e., disidentification from internal experience). This chapter describes the theoretical foundations of the SES-IAT, findings supporting its reliability and validity, its methodology and score computation, and possible future directions for the SES-IAT.