Abstract　 　　
　Signposts of planets in protoplanetary disks produced by disk-planet interactions may provide us a unique way to probe and study the earliest stages of planet formation. I will start with numerical simulations of disk-planet interactions in protoplanetary disks. Particularly, the damping of the density waves excited by planets due to the wave nonlinearity will be discussed, which can result in gap opening in low viscosity disks by low mass planets. Then I will move on to observations of transitional disks, which are protoplanetary disks with central depleted regions (cavities). Several ideas on the formation of transitional disks have been proposed, including gaps opened by planet(s). Recently, Subaru directly imaged a number of such disks at near infrared (NIR) wavelengths (the SEEDS project) with high spatial resolution and small inner working angles. Using radiative transfer simulations, we study the structure of these objects by modeling the NIR images, the SED, and the sub-mm observations from literature (whenever available) simultaneously. Interestingly, we find that in some cases cavities are not present in the scattered light. In such cases we present a new transitional disk model to simultaneously account for all observations, and the model may necessitate the dust filtration mechanism. We have also identified complicated structures, such as spiral arms, in some of these disks, which may indicate the presence of planets. For another group of transitional disks in which Subaru does reveal the cavities at NIR, we focus on the properties of the cavities, and comment on their possible planets origin.