Transthoracic lung ultrasound is ultrasonographic assessment of the lungs, parietal and visceral pleura and the space between these two membranes. It is a part of chest ultrasound examination which also includes assessment of the ribs, intercostal spaces, diaphragm, heart and mediastinum. Lung ultrasound as a diagnostic tool which has been known for 30 years, but its advantages and disadvantages are not commonly known and therefore it is not routinely used by clinicians. In paediatrics, lung ultrasound has become more popular since the first decade of the 21st century. The main advantage of this method is no exposure to ionizing radiation. Transthoracic lung ultrasound can be performed in practically any conditions and repeated several times. What is more, its accessibility, reliability, accuracy and low cost compared to other methods, makes lung ultrasound the great tool for lung assessment. It can be used both for initial diagnosis and monitoring of treatment (bedside examinations are possible). In this paper, we describe the technique of lung ultrasound examination, equipment requirements, normal lung ultrasound appearance, disadvantages of the examination and lung ultrasound findings in the most common paediatric lung diseases, such as: pneumonia, pleural fluid, atelectasis, pneumothorax, bronchiolitis, pulmonary oedema and pulmonary embolism.