Conditions affecting the pulmonary parenchyma are common in veterinary medicine and include traumatic injuries, pulmonary oedema of both cardiac and non-cardiac origin, primary and/or metastatic neoplasia, pneumonia and interstitial lung disease. The focus of this chapter is on pneumonia and interstitial lung disease. Clinical signs of lung diseases are similar, despite different causes, and reflect the alterations in lung mechanics that accompany lung pathology. If the lung parenchyma fills with fluid (oedema) or inflammatory cells (pneumonia), the lungs become less compliant and it is more difficult for the patient to breathe. This is recognized as rapid breathing or as an increased effort. Conditions that cause fluid or pus to leak into the airways or cause primary inflammation or infection in the trachea will cause coughing. Thus, clinical signs are not specific for the disease and the clinician should be alert to considering a number of possibilities when initiating diagnostic testing and therapy. Bacterial pneumonia; Atypical pneumonias; and Interstitial lung disease are all considered.