The mannitol test was developed as an easy-to-use, safe, standardized bronchial challenge test for diagnosing asthma in a wide range of clinical settings. The mannitol test has a moderate sensitivity and will only detect approximately 60% of asthma cases. Hence, a negative mannitol test cannot be used to rule out asthma. The advantage of the mannitol test is a high specificity. In an individual with symptoms suggestive of asthma, a positive test indicates a high likelihood of asthma with ongoing airway inflammation and seems useful for detecting asthma requiring regular anti-inflammatory therapy. In this review, the current knowledge on the usefulness of the mannitol in a clinical setting as well as in clinical trials is presented and outstanding questions on the usefulness of the test are discussed.