BACKGROUND: Pleural infection is a condition that continues to pose a significant challenge to respiratory physicians. We hypothesise that the main barriers to progress include a limited understanding of the pathogenesis, microbiology and role of antibiotics in the pleural space. DATA SOURCE: PubMed was searched for articles related to adult pleural infection using the terms 'pleural infection', 'empyema' and 'parapneumonic'. The search focused on developments in the last ten years, with any older citations only used to display context or lack of progress. Tuberculous pleural infection was excluded. RESULTS: We summarise our latest understanding of the pathogenesis of pleural infection, including recent advances in diagnostics and biomarkers. We discuss our understanding of the pleural microbiome, which differs inherently to that of the lung, and rationalise the current use of antibiotics in treating this condition. 2 Pneumonia vs Pleural Infection 2.1 The pathogenesis of pneumonia Pneumonia can be defined as an infectious process resulting from the invasion and overgrowth of microorganisms in lung parenchyma, breakdown of host defences and provocation of intra-alveolar exudates[8]. The causes can be divided into intrinsic and extrinsic factors. Extrinsic factors include exposure to a causative agent, pulmonary irritants or direct injury. Intrinsic factors are related to the host relating to loss of upper airway reflexes, which allows aspiration of contents and microorganisms from the upper airways into the lung. Underlying disease, loss of mechanical respiratory defences with the use of sedatives, tracheal intubation, and antibiotic treatment are all determinant factors for change in the normal flora of the upper respiratory tract. Once these bacteria find their way to down to the lung parenchyma , a combination of factors may lead to bacterial pneumonia. These include virulence of infecting organism, status of local defences and overall health of the patient. Susceptibility to infection is usually increased in patients of advanced age, or those with chronic disease, due to impairment of the immune response. Dysfunction of defence mechanisms, such as is seen in smokers, patients with chronic obstructive pulmonary disease and tumours, can also lead to greater susceptibility. 2.2 The evolution of the parapneumonic effusion 2.2.1 The exudative phase The interval between aspiration of organisms and the development of pneumonia varies from a few days up to one week. Pneumonia typically begins in dependent lobes at the periphery of the lung, and if untreated, spreads centripetally towards the hilum. If left untreated for the subsequent 2-5 days, a 'simple' parapneumonic effusion will develop and this is known as the "exudative" phase[9]. The development of the initial effusion is due to increased permeability of the pleural membranes in response to inflammation in the underlying lung parenchyma, which is thought to result in transfer of the interstitial fluid across the visceral pleura. Indirect clinical evidence sugg...