Complex breathlessness (breathlessness with an unclear cause or that appears disproportionate to underlying causes) is an important and often neglected condition that presents a challenge to both patients and clinicians. This Monograph aims to provide clinicians at all levels with a comprehensive guide to complex breathlessness. Covering all aspects of this challenging clinical scenario, it will prove a useful resource when faced with a symptomatic breathless patient with no obvious underlying cause or with disproportionate symptoms. Several less commonly considered clinical entities and their associated features are highlighted, and approaches to breathlessness management are suggested in the absence of a clear diagnosis or set of abnormalities on investigation.