This chapter discusses the terminology, stressing the difficulties of the words and concept of medically unexplained symptoms (MUS), and discussing the pros and cons of alternative terms. A fundamental problem with the concept underlying medically unexplained symptoms is the dualism it fosters. The chapter provides the empirical foundation of positive psychobehavioural descriptors, and refers to their suitability as diagnostic criteria in more detail. It has been shown that patients with chronic unexplained symptoms report a negative self-concept of being weak, not tolerating stress and not tolerating any physical challenges. Avoidance of physical activities was the most powerful discriminator between patients with somatic complaints needing medical help and feeling disabled, and those with somatic complaints but without healthcare needs or disability. An important conceptual issue concerns the influence on classification of the psychophysiological models for the experience of disabling bodily symptoms.