Thoracic ultrasonography is now an integral part of equine practice. Thoracic ultrasonography provides the veterinarian with diagnostic information about the lung, pleural cavity, and mediastinum that is readily obtainable in the field. The presence or absence of pleural fluid can be determined, along with its character; the lung affected or most severely affected can be determined and pulmonary atelectasis, consolidation, abscess, and necrosis can be differentiated. The ultrasonographic examination can be used as a screening tool for horses in an outbreak of equine influenza, looking for pulmonary consolidation, or in foals with suspected Rhodococcus equi infection, looking for pulmonary abscesses. An ultrasound-guided biopsy can be performed to further characterize the pulmonary pathology in horses when the sonographic findings are consistent with several differential diagnoses (pulmonary fibrosis, metastatic neoplasia, and granulomatous pneumonia). The information obtained on the initial thoracic ultrasound examination can be used to formulate a prognosis for horses with pleuropneumonia and pulmonary abscesses.