The review is devoted to a description of the possibilities afforded by transmission electron microscopy for the investigation of magnetic structures of thin ferromagnetic films, and presents experimental data on magnetic films obtained by using a transmission electron microscope. We consider and compare different microscope operating regimes for observing the magnetic structure. The formation of the contrast of the magnetic structures in the most widely used modes, namely the defocusing mode and the small-angle electron diffraction mode, are considered in detail. Particular attention is paid to obtaining data on the domain walls and the fine structure of the magnetization inside the domains. Different methods of measuring the widths of the boundaries are compared, and the possible sources of errors are discussed. The prospects for using ultra-high voltage electron microscopes for the investigation of thicker films are noted. Experimental data are given on the investigation of domain configurations in single-crystal foils and in polycrystalline films, and the dependences of the magnetic parameters of the film on the structure and on the conditions of obtaining the films are discussed. Results of an investigation of a quasistatic and pulsed remagnetization of magnetic films are described.