Diagnosis of a sleep disorders is multimodal. An overview is presented in this review. The medical history leads to a tentative diagnosis supported by questionnaires, sleep diary and objective methods. Examination may reveal upper airway problems in a patient suspected to have obstructive sleep apnoea or rigidity in an elderly patient with shouting during sleep, suggestive of rapid eye movement sleep behaviour disorder. The choice of diagnostic sleep test is based on the tentative diagnosis. Other tests (e.g., lumbar puncture, brain scan) may be indicated. Wearables pose the advantage of documenting the patients' habitual sleep and circadian rhythm.