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The work presented in this thesis is a study of the reversal mechanisms of the magnetic layers within spin-valve materials and related films. Spin-valves display the phenomenon of giant magnetoresistance (GMR) and are now being utilised as magnetoresistive read heads in commercial applications. A spin-valve typically consists of two ferromagnetic layers separated by a thin spacer layer (eg Cu in the range 2-10nm). One of the ferromagnetic layers is "pinned" or fixed in direction by exchange coupling to an antiferromagentic layer such as FeMn or IrMn. This shifts the hysteresis loop by a few hundred Oersted (Oe), and so only the other "free" ferromagnetic layer can reverse under the influence of the comparatively small magnetic field from a passing tape or disk. This reversal corresponds to a change between a parallel 'low' resistance state to an antiparallel 'high' resistance state under the application of a magnetic field of ≈10-150e. The reversal mechanisms that take place in the "free" and "pinned" magnetic layers in a range of spin-valve materials have been studied using Lorentz modes of transmission electron microscopy. These results form the bulk of this thesis. The effect of Molybdenum impurity on the magnetoresi stance of Ni80Fe20 is considered as a secondary topic, and only chapter 6 is given over to these results. The first chapter introduces the basic concepts of ferromagnetism, magnetoresistance, and its application in magnetic storage technology. Particular emphasis is given to the various energy contributions that are present in thin magnetic films. This leads to the concept of domains and domain walls. Anisotropic magnetoresistance (AMR) and giant magnetoresistance (GMR) are introduced. These phenomena have enabled the production of many thin film sensors for applications including magnetic storage technology. Spin-valves are then discussed, reference being given to their application as a sensor in magnetic read-head assemblies. The possible commercial benefits of using GMR based devices for magnetic storage applications are highlighted. Transmission electron microscopy was the primary tool used to investigate the materials discussed in this thesis. Thus chapter 2 is devoted to discussion of the instrumentation and techniques employed. An overview of the important parts of a TEM is introduced. including the electron gun and microscope column. The aberrations which limit the resolution of the microscope are mentioned before the techniques used to image structural properties are presented. Particular attention is then paid to the Fresnei, Foucault and Low angle diffraction (LAD) imaging modes of Lorentz electron microscopy which are used to investigate the magnetic structures in thin films. Finally, the methods used to apply magnetic fields in-situ are discussed. Chapter 3 begins with a review of spin-valves detailing parameters such as the magnetic layer configuration, anisotropy arrangement, free layer reversal and deposition technique used for the samples investigated in this thesis. Structural properties such as the average grain size of the spin-valves are investigated by bright and dark field imaging. Diffraction studies allow some compositional data and the level of texture to be evaluated. The main body of results in this chapter concentrates on the magnetisaton reversal mechanism of the free layer in a range of FeMn-biased spin-valves with parallel anisotropy. The thickness of the copper spacer layer (and hence the strength of the interlayer coupling strength), is varied in the range 2-10nm, as is the angle of applied field, 0, with respect to the biasing direction. Fresnel imaging and LAD studies reveal there to be 3 modes of reversal that are possible in the free layer depending on the interlayer coupling strength and field orientation. Two of the observed modes involve a combination of magnetisation rotation and domain assisted processes while the third involves coherent rotation of magnetisation alone. The boundary between the modes of reversal was shown to be indistinct, and involved a free layer reversal that proceeded by an increasing amount of magnetisation rotation as ? was decreased. This was accompanied by the formation of an increasing density of low angle, low mobility domain walls. The observed modes of reversal are presented on a phase diagram and shown to be in many ways consistent with a coherent rotation model by Labrune. The main discrepancy between the model and observed modes of reversal is that domains are forbidden in the model. The chapter concludes with a brief study of a spin valve in a crossed anisotropy arrangement.