Critical ultrasound, apparently a recent field, is in fact the outcome of a slow process, initiated since 1946. The lung was traditionally not considered as part of ultrasound, yet we considered its inclusion as a priority in our definition of critical ultrasound. Acute respiratory failure is one of the most distressing situations for the patient. An ultrasound approach of this disorder - the BLUE-protocol allows rapid diagnosis. Its main features will be described. Each kind of respiratory failure provides a particular ultrasound profile. In this difficult setting, initial mistakes are frequent. The BLUE-protocol proposes a step-by-step approach for making accurate diagnosis. By combining three signs with binary answer (anterior lung sliding, anterior lung-rockets), with venous analysis when required, seven profiles are generated, yielding a 90.5% accuracy. This rate is highly enhanced when simple clinical and laboratory data are considered. The BLUE-protocol can be achieved in three minutes, because the use of an intelligent machine, a universal probe, and standardized points allow major time-saving. Lung ultrasound in the critically ill was long available. In a domain where everything must be fast and accurate, the BLUE-protocol can play a major role in the diagnosis of an acute respiratory failure, usually answering immediately to questions where only sophisticated techniques were hitherto used.