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This thesis is concerned with the use of a dedicated scanning transmission electron microscope (STEM) to investigate the physical and magnetic microstructure of thin film specimens. The materials investigated were ferromagnetic and some of them are considered as possible media for high density perpendicular magnetic recording. The first chapter introduces the concepts of ferromagnetism and the energy contributions which determine the domain structures in thin films. The requirements for magnetic recording media are also discussed in this chapter and the reasons for interest in perpendicular, rather than longitudinal recording, as a means of achieving high information storage densities are discussed. Chapter 2 describes the STEM used in this work, which was an extended V. G. Microscopes HB5, and then goes on to explain image formation in a STEM. It is shown that the use of a non-standard detector with an asymmetric response function in the STEM allows direct information on magnetic structures within a thin film to be obtained (the differential phase contrast imaging mode). Furthermore it is seen that the very small source size of the field emission gun in the HB5 allows this information to be obtained with a high spatial resolution, and that the sequential nature of image formation in the STEM allows this information to be acquired digitally. The remainder of the chapter deals with the microscope conditions used for, and spatial resolution available with, x-ray and EELS microanalysis. Chapter 3 demonstrates the use of the STEM to characterise fully the physical and magnetic microstructure of a large grain polycrystalline cobalt foil. In this chapter it is demonstrated that electron diffraction can be used to determine the crystallography of the foil, the thickness can be determined by measuring the deflection of the electron beam as it passes through the foil, and the magnetic structure and domain wall profile can be determined using differential phase contrast imaging in the STEM. In Chapter 4 the magnetic stray fields from the cobalt foil of Chapter 3 are investigated, as are those from written tracks on a longitudinal magnetic recording medium. It is seen that the measured stray fields from the simple domain structure of the cobalt foil are in good agreement with theoretical calculations. In Chapter 5 the techniques demonstrated in Chapter 3 are applied to planar polycrystalline Co-Cr films with artificial superstructures and to CoCr alloy films. Both materials were grown so as to have a preferential crystal orientation, with a c-axis texture, in the alloys and in the Co layers of the artificial superstructure films, along the film normal. The magnetic structure observed in these films indicated that this c-axis texture had given rise to a degree of perpendicular magnetic anisotropy. In Chapters 6 and 7 cross-sections of single and double layer perpendicular magnetic recording films are investigated. In Chapter 6, elemental segregation of Cr to individual column boundaries within the columnar microstructure is demonstrated using x-ray microanalysis. When the effects of beam broadening are considered it is seen that the column boundaries are likely to be non-magnetic. In Chapter 7 the magnetic structure of the cross-sections is investigated. Although no unambiguous magnetic contrast is revealed in the CoCr layers, the magnetic structure in the backing layers, and the stray fields from the cross-sections show that there is very small scale magnetic structure within the CoCr layer. Chapter 8 is a summary of the results obtained, and contains suggestions for future investigations of magnetic recording materials. Three of the four Appendices are concerned with computer processing of STEM images, and one explains the use of the free lens controller on the CTEM used in this work for preliminary investigations.