X-ray photoelectron spectroscopy (XPS) is a widely used standard method in the analysis of surfaces. In its conventional form a vacuum technique, it was adapted to samples in ambient pressures up to around 1 mbar already in the 1970s. However, it is only during the past 15 years that ambient pressure x-ray photoelectron spectroscopy (APXPS) has become a standard in situ measurement technique, as a result of the advent of third-generation synchrotron light sources and improved electron energy analysers. We will provide an introduction to APXPS, its technical realisation, and, using a number of case studies, its capabilities for the identification of surface species, structure, and reactions. In particular, we will discuss how we have used APXPS in the in situ study of the atomic layer deposition (ALD) of light and heavier element materials. The presentation will also include an overview of APXPS studies of Mg and other materials.