The diagnosis of obstructive sleep apnea syndrome (OSAS) requires the combined assessment of relevant clinical features and the objective demonstration of abnormal breathing during sleep, and current evidence indicates that attempts to base the diagnosis of the clinical syndrome on either aspect alone are unreliable. The present review discusses the clinical assessment of patients with suspected OSAS and also the potential added value of structured questionnaires and clinical prediction models that seek to improve the diagnostic value of clinical assessment from the formalized evaluation of selected clinical features. While the traditional "gold standard" for objective assessment is laboratory-based polysomnography, there is growing evidence that limited sleep studies focused on respiratory and cardiac variables are adequate in most cases, and are particularly suited to home-based assessment. The choice between home versus sleep laboratory studies should be decided by taking into account resource limitations and the clinical index of suspicion for OSAS. At present, patients with either a low or high clinical index of suspicion for OSAS appear most suited to home-based investigation, whereas those with intermediate levels of clinical suspicion, or who present with atypical clinical features, may best be assessed by full polysomnographic studies in the first instance.