The study of spin dynamics in organic semiconductors is interesting not only for the exceptionally long spin lifetimes present in organics, but also because it can act as a probe of charge dynamics. Tools and techniques developed over the past 15 years aim to probe this connection, focusing on how the inexorable relationship between charge and spin can be leveraged in device design. Of particular interest is how quantum spin can be used to enhance or inhibit charge transport, and how spin itself can be used as a means of information transfer. Tied to all of these ideas is that spin can be altered via its couplings to the local environment. While this very property is what permits such techniques in the first place, it also introduces the challenge of preventing unwanted changes in spin. Much research has gone into understanding the coupling mechanisms that influence spin, and how to tailor molecules for device-specific goals.

This thesis uses magnetic resonance techniques to explore spin dynamics in detail. Chapter 1 is an introduction to the topic, while Chapters 2, 3, and 4 provide the theoretical and experimental background necessary to analyze the data presented. Chapter 5 is the first experimental chapter. The measurements described within are designed to probe spin relaxation at temperatures between 5 and 300 K, and emphasize especially those occurring between 150 and 300 K --- a regime not currently well modeled. By using ambipolar semiconductors, the research links the spatial extent of the wavefunction and its position along the semiconductor backbone to relaxation via nuclear torsion. This is the first experimental evidence for the relaxation model at `high temperatures' (150 - 300 K) first proposed by our group in 2019. Chapter 6 continues work on ambipolar systems. In this case, electrically detected magnetic resonance is used to understand how spin affects electrical currents flowing through devices. We sweep through all regimes of device operation (electron-only, ambipolar, hole-only) to provide evidence for both bipolaron-inhibited and recombination-assisted currents. Chapter 7 focuses on small-molecule crystalline systems rather than amorphous polymers. In rubrene --- a model system for organic field-effect transistors --- we report microsecond-long relaxation times from 15 K all the way to room temperature. In the framework of the relaxation theory developed in Chapter 2, we show how such long and stable relaxation times are possible, and how spin-spin interactions manifest.