Protoplanetary disks are the sites of planet formation but how dust particles evolve into planets in the disks and finally form as a planetary system is a long-standing mystery. In this talk, I will review our understanding of protoplanetary disks in an observational viewpoint and show recent NIR and mm/sub-mm observational results in terms of disk demographics, geometrical structures, dust properties, dust trapping and grain growth and polarization. In particular, the disks with holes or gaps with mass transfer seen as a steamer crossing the disk that many researchers have posited as the signpost of planets will be stressed. I will also introduce chemical composition of the disk including pre-biotic organic materials. I will conclude by showing future directions for our study of planet formation and new observational instruments and operations in the coming years that may provide the answer for the key questions.
The disks that orbit young stars are the essential conduits and reservoirs of material for star and planet formation. Their structures, meaning the spatial variations of the disk physical conditions, reflect the underlying mechanisms that drive those formation processes. Observations of the solids and gas in these disks, particularly at high resolution, provide fundamental insights on their mass distributions, dynamical states, and evolutionary behaviors. Over the past decade, rapid developments in these areas have largely been driven by observations with the Atacama Large Millimeter/submillimeter Array (ALMA). This review highlights the state of observational research on disk structures, emphasizing the following three key conclusions that reflect the main branches of the field.