Why are planets and flybys important for debris discs?

Debris discs are belts of planetesimals and dust, like the Asteroid Belt and Kuiper Belt in our Solar System. These discs are a fundamental component of planetary systems, and many such discs have been resolved around other stars. Debris discs show a broad diversity of locations, shapes and features, and we do not understand the origin of this diversity. However, these discs mustbe telling us something about the architecture, formation and evolution of planetary systems.One reason that debris discs are important is that they contain imprints of historical dynamical interactions, which can persist long after the events that caused them. For example, planets can gravitationally interact with debris, and this interaction generates disc features like gaps, warps and clumps. Similarly, stellar flybys can produce long-lasting debris structures in the outer regions of planetary systems. Now, with the arrival of new dynamical theory and observational capabilities, we are extremely well placed to interpret the clues that debris discs are giving us.In this talk, I will summarise what planets or flybys would do to debris discs. In particular, I will focus on the observable signatures that these interactions would leave on discs. I will describe why conventional debris models often rely on such interactions having occurred, and what it would mean for debris science if this assumption proved incompatible with observations. I will also discuss how upcoming theory and observations may finally answer some key questions about debris and planetary systems. A wealth of new debris-disc data will arrive over the next few years, and as a community we must be prepared to interpret it.