This thesis aims to develop transmission electron microscopy (TEM) techniques to study magnetic nanomaterials in three dimensions (3D). At the nanoscale, magnetic materials exhibit various intriguing behaviours such as topological protection in skyrmions (which may be used in next-generation computers), vortex states in nanorings (which are being developed to treat cancer), and long-term stability in paleomagnetic samples (which allow researchers to study the formation of planetary bodies in the solar system). Many of these behaviours are intrinsically 3D in nature, and so a 3D characterisation method is required to fully understand them. This thesis explores how magnetic imaging in the TEM can be combined with electron tomography to provide a tool for studying magnetic nanomaterials in 3D.