 Patients with breathlessness and respiratory failure represent a significant proportion of emergency admissions to hospital and commonly require urgent treatment with limited information available to guide the clinician. The range of diseases that present with breathing difficulties is broad e.g. pneumonia, heart failure, pulmonary embolus and difficult to differentiate in patients who often have multiple medical problems. This leads to non-specific treatment in the face of diagnostic uncertainty with the associated risks of treatment complications, increased morbidity and mortality, and distress for patients and relatives. It is in the assessment of these patients with acute respiratory failure where thoracic ultrasound might be of greatest benefit and which this research is designed to address. 