AFM's path to atomic resolution

We review progress in improving the spatial resolution of atomic force microscopy (AFM) under vacuum. After an introduction to the basic imaging principle and a conceptual comparison to scanning tunneling microscopy (STM), we outline the main challenges of AFM as well as the solutions that have evolved in the first 20 years of its existence. Some crucial steps along AFM's path toward higher resolution are discussed, followed by an outlook on current and future applications.