In this chapter, we propose to review and discuss the use of waveguides in spintronic devices and other magnetic systems as well as some emergent phenomena linked to ultrafast magnetic dynamics. Spintronic devices are expected to replace the recent nanoelectronic memories and sensors due to their efficiency in energy consumption and functionality with scalability. In the field of spintronic devices, it is required the development of magnetic thin films with a wide range of magnetic properties. To achieve the characterization of magnetic thin films, several techniques are very useful, particularly in studying magnetization dynamics described by the phenomenological Landau-Lifshitz-Gilbert equation. These techniques are developed based on key phenomena such as spin pumping, ferromagnetic resonance, the recently observed terahertz oscillations and ultrafast switching. The coupling of waveguides and microwave generators to produce radio-frequency magnetic fields and integrate them with magnetic thin films is crucial to characterize the magnetization dynamics.