Since the 1978-79 Skylab Workshop on Solar Active Regions, significant progress has been made in the diagnostic and modeling of active solar structures. This paper review the status of our knowledge of the physical properties of active regions in the photosphere, chromosphere and transition zone, as inferred from observations. Plages are the main active region which we study. A comparison with quiet Sun and some other solar structures is made in order to in order to emphasize the spedificity of plages properties. To conclude we present some requirements for new observations and theoretical developments to improve our understanding of the physical structure of solar active regions.