Aims : to summarise the current knowledge of the many clinical faces of obstructive sleep apnoea (OSA) characterised by a variable degree of daytime sleepiness, gender differences of OSA symptoms and differences in the presence of comorbid insomnia, resulting in various comorbidities. It will cover the heterogeneity of OSA and its cognitive outcomes, especially in terms of attention and executive functions, and the potential bidirectional relationship with dementia, focusing on early identification, prevention and treatment strategies. The physiological and biochemical mechanisms connecting OSA with metabolic syndrome and diabetes, clinically meaningful phenotyping, improved prognosis and personalised treatment will be covered. The impact of the severity of OSA on the risk and outcomes of cardiovascular diseases and evidence of the impact of OSA treated with or without continuous positive airway pressure (CPAP) on morbidity and mortality outcomes will be discussed. The challenges facing sleep medicine regarding the important effect of CPAP on patient outcomes and the role of compliance will be highlighted.