Transition disks: the observational revolution from SEDs to imaging

Protoplanetary disks surrounding young stars are the birth place of planets. Of particular interest are the transition disks with large inner dust cavities of tens of au, hinting at the presence of massive companions. These cavities were first recognized by a deficit in their Spectral Energy Distribution (SED), later confirmed by millimeter interferometry observations. The Atacama Large Millimeter/submillimeter Array (ALMA) has truly revolutionized the field of spatially resolved imaging of protoplanetary disks in both dust and gas, providing important hints for the origin of gaps and cavities. At the same time, new types of substructures have been revealed. Also infrared observations show a large range of substructures both in resolved imaging, interferometry and spectroscopy. Since the last review paper of transition disks (Protostars and Planets VI), a huge amount of data has been taken, which led to numerous new insights in the origin of transition disks. In this review I will summarize the observational efforts from the past decade, compare their insights with the predictions from SED modeling, analyze the properties of the transition disk population and discuss their role in general disk evolution.