When physicists began to explore the world of the atom more closely, in an attempt to understand its structure and the laws governing its behavior, they soon ran into serious difficulties. Our intuitive concepts, based on our everyday experience of the macroscopic world around us, proved to be totally erroneous on the atomic scale ; the atom was incomprehensible within the framework of classical physics. In order to unravel these new mysteries, it was necessary, after much trial and error, to develop entirely new concepts - the concepts of quantum mechanics. What are the main steps that led us to this modern vision of the atom ? What is the current state of atomic physics ? What contribution has it made to the development of our knowledge, and where is it heading ?