Chest ultrasonography can be a useful diagnostic tool for respiratory physicians. It can be used to complete and widen the general objective examination also in emergency situations, at the patient's bedside. The aim of this document is to promote better knowledge and more widespread use of thoracic ultrasound among respiratory physicians in Italy. This document I is focused on basic knowledge of chest ultrasonography technique, physical basis, aims and characteristics, fields of application. Document I shows how chest ultrasonography can be useful to detect and monitor pleural diseases, pleural effusions and pneumothorax and how it can assess diaphragmatic kinetics and pathologies.