Abstract:The observational features like gaps and spirals in protoplanetary disks have been postulated as signposts of embedded planets.  In this talk, I will first explain to you how we discover the gas gaps/rings in the HL Tau disk, which has been well-known for its striking  dust ringlike structures. Then I will summarize our linear result of wave excitation and propagation due to planet-disk interactions and  touch upon its implication for the spirals seen on the surface of a protoplanetary disk.