The majority of young low-mass stars are surrounded by optically thick accretion disks, consisted of large reservoirs of gas and dust out of which planetary systems eventually form. In the recent years, many protoplanetary disks have been identified as the so-called "transition disks", meaning that they harbor large inner cavities. The sizes of inner hole range from a few to more than 70 AU. More interestingly, recent ALMA observations reveal that large-scale asymmetries exist in such transition disks. The origin of those structures and how they are related to planet formation, however, remains unclear. In this talk we will give an introduction to the disk instabilities, in particular the Rossby wave/vortex instability, that might be responsible for the asymmetries.  We will discuss specific scenarios that this instability can lead to asymmetries in disks. By carrying out extensive simulations and comparing them with available simulations, we are gaining better understanding of the disk structure/properties and their connections to planet formation.  Implications for future observations will be explored as well.