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I will give an overview of the mechanics of flyby encounters and how these unbound perturbers can affect the structure of protoplanetary discs. Depending on the density of stellar clusters, stars with gaseous and dusty discs have up to a ~ 30% chance to encounter a flyby event within the first million years. The perturber will excite spiral arms during these flyby events. The spirals have a finite lifetime since the perturber is unbound. I simulate a parabolic encounter interacting with a gaseous protoplanetary disc utilizing three-dimensional smoothed particle hydrodynamical simulations. I find that the spirals can survive even when the perturber has passed periastron and is no longer interacting with the disc. Analyzing the dynamics of these excited spirals can shed light on observations of spirals in protoplanetary discs with no observed companion. *Link to the colloquium: http://meet.google.com/kyg-mjsu-eja