Respiratory muscle weakness may be the sole cause of dyspnea or may aggravate dyspnea due to another respiratory disease, and is often difficult to recognise clinically. The assessment of respiratory muscles should follow a graded approach using tests of increasing complexity. Clinical examination should look for dyspnea, orthopnea, morning headache, daytime somnolence, fatigability, tachypnea, abdominal, or rib cage paradox, and amyotrophy. Imaging is useful in diagnosing diaphragmatic paralysis using chest radiograph, fluoroscopy or ultrasound. In cases of moderate to severe respiratory muscle weakness, lung volumes show reduced vital capacity and total lung capacity. Measuring the change in vital capacity from sitting to supine position is useful since it shows a 25-50% fall in cases of diaphragmatic paralysis. The specific and classical tests of respiratory muscle strength are maximum inspiratory and expiratory pressures (MIP and MEP) sustained during one second against near complete occlusion. Sniff nasal inspiratory pressure (SNIP) is a new and easier test of inspiratory muscle strength. Normal values obtained with these simple tests rule out clinically significant respiratory muscle weakness. In case of doubt, more complex and invasive tests can be used such as transdiaphragmatic pressure and magnetic stimulation of the phrenic nerves.