In a first part, I will briefly survey how a magnetic field can be used in order to focus an electron beam and then how a transmission electron microscope works. Recent experimental developments such as the correction of the spherical aberration of electron lenses and the resulting atomic resolution will be showed. I will then present how this techniques are now widely used in the order to solve various problematics such as the imaging of nanostructures (from the industrial oriented characterisation of multilayers to the imaging of nanoparticles into organic matter) or the structural refinement of crystalline or defective systems (such as bonding at interfaces or the topological description of defects in functionalised nanotubes). Finally I will discuss how combining electron spectroscopy and microscopy, physical or chemical properties such as the oxidation state of transition metal oxide nanoparticles or the plasmonic properties of metallic clusters can be investigated.